:root {
    --bg: #eef2f7;
    --surface: #ffffff;
    --card-bg: #ffffff;
    --card-border: #e2e8f0;
    --header-bg: rgba(255, 255, 255, 0.82);
    --text: #1f2933;
    --muted: #5b6b7b;
    --border: #e0e4e8;
    --primary: #2563eb;
    --accent: #9333ea;
    --primary-contrast: #ffffff;
    --error-bg: #fef2f2;
    --error-text: #b91c1c;
    --cell-size: clamp(50px, 5vw, 64px);
    --gap: 4px;
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 18px;
    --shadow-xs: 0 1px 2px rgba(16, 24, 40, 0.05), inset 0 1px 0 rgba(255, 255, 255, 0.5);
    --shadow-sm: 0 2px 6px rgba(16, 24, 40, 0.06);
    --shadow-md: 0 10px 24px rgba(16, 24, 40, 0.14);
    --shadow-lg: 0 24px 48px -14px rgba(16, 24, 40, 0.22);
    --hover-scale: 1.06;
    --transition-fast: 180ms ease;
    --transition-smooth: 260ms cubic-bezier(0.22, 0.61, 0.36, 1);
    --cell-text: #1f2933;
    --cell-muted: #475569;
    --glow-a: rgba(37, 99, 235, 0.09);
    --glow-b: rgba(147, 51, 234, 0.07);
    --bg-dots: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='26' height='26'%3E%3Ccircle cx='2' cy='2' r='1' fill='%2364748b' fill-opacity='0.16'/%3E%3C/svg%3E");
}

[data-theme="dark"] {
    --bg: #0b1220;
    --surface: #1a2332;
    --card-bg: #141d2b;
    --card-border: #26324a;
    --header-bg: rgba(11, 18, 32, 0.78);
    --text: #e2e8f0;
    --muted: #94a3b8;
    --border: #2c3a52;
    --primary: #60a5fa;
    --accent: #a78bfa;
    --primary-contrast: #ffffff;
    --error-bg: #3b0d0d;
    --error-text: #fca5a5;
    --shadow-xs: 0 1px 2px rgba(0, 0, 0, 0.4), inset 0 1px 0 rgba(255, 255, 255, 0.06);
    --shadow-sm: 0 2px 6px rgba(0, 0, 0, 0.35);
    --shadow-md: 0 12px 28px rgba(0, 0, 0, 0.5);
    --shadow-lg: 0 28px 56px -14px rgba(0, 0, 0, 0.6);
    --glow-a: rgba(96, 165, 250, 0.16);
    --glow-b: rgba(167, 139, 250, 0.13);
    --bg-dots: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='26' height='26'%3E%3Ccircle cx='2' cy='2' r='1' fill='%2394a3b8' fill-opacity='0.14'/%3E%3C/svg%3E");
}

*,
*::before,
*::after {
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
}

html,
body {
    overflow-x: clip;
}

body {
    margin: 0;
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
    line-height: 1.5;
    color: var(--text);
    background-color: var(--bg);
    background-image:
        radial-gradient(1100px 520px at 10% -8%, var(--glow-a), transparent 60%),
        radial-gradient(900px 480px at 100% 6%, var(--glow-b), transparent 55%),
        var(--bg-dots);
}

@media (min-width: 768px) {
    body {
        background-attachment: fixed;
    }
}

.container {
    max-width: 1340px;
    margin-inline: auto;
    padding-inline: 20px;
}

/* Header */
.header {
    position: sticky;
    top: 0;
    z-index: 10;
    padding: 14px 0;
    background-color: var(--header-bg);
    -webkit-backdrop-filter: blur(14px) saturate(140%);
    backdrop-filter: blur(14px) saturate(140%);
    border-bottom: 1px solid var(--border);
    box-shadow: var(--shadow-sm);
}

@supports not ((backdrop-filter: blur(1px)) or (-webkit-backdrop-filter: blur(1px))) {
    .header {
        background-color: var(--surface);
    }
}

.header__inner {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 16px;
    flex-wrap: wrap;
}

.header__title {
    margin: 0;
    font-size: 1.4rem;
    font-weight: 800;
}

@supports ((-webkit-background-clip: text) or (background-clip: text)) {
    .header__title {
        background-image: linear-gradient(115deg, var(--primary), var(--accent));
        -webkit-background-clip: text;
        background-clip: text;
        color: transparent;
    }
}

.header__subtitle {
    margin: 2px 0 0;
    font-size: 0.85rem;
    letter-spacing: 0.02em;
    color: var(--muted);
}

.header__actions {
    display: flex;
    align-items: center;
    gap: 10px;
}

.learn-link {
    display: inline-flex;
    align-items: center;
    gap: 6px;
    padding: 9px 14px;
    font-size: 0.88rem;
    font-weight: 600;
    font-family: inherit;
    color: var(--primary);
    background-color: var(--surface);
    border: 1px solid var(--border);
    border-radius: 999px;
    text-decoration: none;
    box-shadow: var(--shadow-xs);
    transition: border-color var(--transition-fast), transform var(--transition-fast), box-shadow var(--transition-fast), background-color var(--transition-fast);
}

.learn-link:hover {
    border-color: var(--primary);
    transform: translateY(-1px);
    box-shadow: var(--shadow-sm);
}

.theme-toggle {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 42px;
    height: 42px;
    font-size: 1.25rem;
    background-color: var(--surface);
    border: 1px solid var(--border);
    border-radius: 50%;
    cursor: pointer;
    box-shadow: var(--shadow-sm);
    transition: border-color var(--transition-fast), transform var(--transition-smooth), box-shadow var(--transition-fast), background-color var(--transition-fast);
}

.theme-toggle:hover {
    border-color: var(--primary);
    transform: translateY(-1px) scale(1.05);
    box-shadow: var(--shadow-md);
    background-color: color-mix(in srgb, var(--surface) 80%, var(--primary) 20%);
}

.theme-toggle:active {
    transform: scale(0.92);
}

/* Main */
.main {
    padding: 24px 0 40px;
    min-height: calc(100vh - 78px);
}

/* Periodic view (table mode / explorer mode) */
.periodic-view {
    position: relative;
}

.periodic-view.is-table {
    padding: 20px;
    background-color: var(--card-bg);
    border: 1px solid var(--card-border);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-lg);
}

.table {
    width: 100%;
    margin: 0 auto;
    display: grid;
    grid-template-columns: repeat(18, minmax(0, 1fr));
    grid-auto-rows: var(--cell-size);
    gap: var(--gap);
}

.element {
    position: relative;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 3px 2px;
    border: 1px solid rgba(15, 23, 42, 0.12);
    border-radius: var(--radius-sm);
    background-color: var(--cat-bg, var(--surface));
    color: var(--cell-text);
    cursor: pointer;
    overflow: hidden;
    box-shadow: var(--shadow-xs);
    transition: transform var(--transition-smooth), box-shadow var(--transition-smooth), border-color var(--transition-fast), opacity var(--transition-fast);
}

.element::after {
    content: "";
    position: absolute;
    inset: 0;
    background: linear-gradient(105deg, transparent 40%, rgba(255, 255, 255, 0.45) 50%, transparent 60%);
    transform: translateX(-130%);
    pointer-events: none;
}

.element:hover {
    transform: translateY(-3px) scale(var(--hover-scale));
    border-color: var(--primary);
    box-shadow:
        0 12px 26px rgba(16, 24, 40, 0.18),
        0 0 22px color-mix(in srgb, var(--cat-bg) 65%, transparent);
    z-index: 1;
}

.element:hover::after {
    transform: translateX(130%);
    transition: transform 0.65s ease;
}

.element__number {
    font-size: 0.6rem;
    line-height: 1;
    color: var(--cell-muted);
}

.element__symbol {
    font-size: 1.4rem;
    font-weight: 800;
    line-height: 1.15;
    letter-spacing: 0.01em;
}

.element__name {
    max-width: 100%;
    font-size: 0.55rem;
    line-height: 1.2;
    color: var(--cell-muted);
    white-space: nowrap;
    overflow: hidden;
    text-overflow: ellipsis;
}

.element__mass {
    font-size: 0.5rem;
    line-height: 1.2;
    color: var(--cell-muted);
}

@keyframes element-in {
    from {
        opacity: 0;
        transform: translateY(10px) scale(0.97);
    }
    to {
        opacity: 1;
        transform: translateY(0) scale(1);
    }
}

.element {
    animation: element-in 380ms cubic-bezier(0.22, 0.61, 0.36, 1) backwards;
}

/* Explorer mode (tablet + mobile) */
.explorer {
    display: grid;
    gap: 22px;
}

.explorer-group__title {
    display: flex;
    align-items: center;
    gap: 8px;
    margin: 0 0 10px;
    font-size: 0.95rem;
    font-weight: 800;
    letter-spacing: 0.02em;
}

.explorer-group__title::before {
    content: "";
    flex-shrink: 0;
    width: 6px;
    height: 18px;
    border-radius: 3px;
    background: linear-gradient(160deg, var(--primary), var(--accent));
}

.explorer-group__grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(min(140px, 100%), 1fr));
    gap: 8px;
}

.element--card {
    min-height: 92px;
    padding: 10px 6px;
    border-radius: var(--radius-md);
}

.element--card .element__number {
    position: absolute;
    top: 6px;
    left: 8px;
    font-size: 0.68rem;
}

.element--card .element__symbol {
    font-size: 1.6rem;
}

.element--card .element__name {
    font-size: 0.74rem;
    padding-inline: 4px;
}

.element--card .element__mass {
    display: none;
}

.periodic-view.is-explorer ~ .series-note {
    display: none;
}

@media (prefers-reduced-motion: reduce) {
    html {
        scroll-behavior: auto;
    }

    .element {
        animation: none;
    }

    .element,
    .element::after,
    .theme-toggle,
    .legend__chip,
    .series-note__item,
    .element--card,
    .learn-chip,
    .learn-link,
    .speak-button,
    .learn__steps li {
        transition: none !important;
    }

    .element:hover,
    .element--card:hover,
    .learn-chip:hover,
    .learn-link:hover {
        transform: none;
    }

    .element:hover::after {
        transform: translateX(-130%);
    }

    body {
        background-attachment: scroll;
    }
}

.cat-alkali-metal { --cat-bg: #fca5a5; }
.cat-alkaline-earth-metal { --cat-bg: #fdba74; }
.cat-transition-metal { --cat-bg: #fcd34d; }
.cat-post-transition-metal { --cat-bg: #a7f3d0; }
.cat-metalloid { --cat-bg: #5eead4; }
.cat-diatomic-nonmetal { --cat-bg: #7dd3fc; }
.cat-polyatomic-nonmetal { --cat-bg: #93c5fd; }
.cat-noble-gas { --cat-bg: #c4b5fd; }
.cat-lanthanide { --cat-bg: #f9a8d4; }
.cat-actinide { --cat-bg: #f0abfc; }
.cat-unknown { --cat-bg: #cbd5e1; }

/* Lanthanide / Actinide series note */
.series-note {
    display: flex;
    flex-wrap: wrap;
    gap: 10px;
    justify-content: center;
    margin: 18px auto 0;
}

.series-note__item {
    display: inline-flex;
    align-items: center;
    gap: 7px;
    padding: 6px 14px;
    font-size: 0.82rem;
    color: var(--text);
    background-color: var(--surface);
    border: 1px solid var(--border);
    border-radius: 999px;
    box-shadow: var(--shadow-xs);
}

.series-note__swatch {
    width: 13px;
    height: 13px;
    border: 1px solid rgba(15, 23, 42, 0.18);
    border-radius: 4px;
    background-color: var(--cat-bg, var(--surface));
}

/* Legend */
.legend {
    display: flex;
    flex-wrap: wrap;
    gap: 10px;
    justify-content: center;
    max-width: 100%;
    margin: 20px auto 0;
    padding: 16px;
    background-color: var(--card-bg);
    border: 1px solid var(--card-border);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-sm);
}

.legend__chip {
    display: inline-flex;
    align-items: center;
    gap: 7px;
    padding: 6px 13px;
    font-size: 0.8rem;
    font-family: inherit;
    color: var(--text);
    background-color: var(--surface);
    border: 1px solid var(--border);
    border-radius: 999px;
    box-shadow: var(--shadow-xs);
    cursor: pointer;
    transition: transform var(--transition-fast), border-color var(--transition-fast), box-shadow var(--transition-fast), background-color var(--transition-fast);
}

.legend__chip.is-active {
    border-color: var(--primary);
    background-color: color-mix(in srgb, var(--primary) 12%, var(--surface));
    box-shadow: 0 0 0 2px color-mix(in srgb, var(--primary) 30%, transparent);
}

.legend__chip:hover {
    transform: translateY(-1px);
    border-color: var(--primary);
    box-shadow: var(--shadow-sm);
}

.legend__swatch {
    width: 13px;
    height: 13px;
    border: 1px solid rgba(15, 23, 42, 0.18);
    border-radius: 4px;
    background-color: var(--cat-bg, var(--surface));
}

/* Error message */
.error-message {
    max-width: 480px;
    margin: 48px auto;
    padding: 20px;
    text-align: center;
    background-color: var(--error-bg);
    border: 1px solid var(--error-text);
    border-radius: var(--radius-md);
    box-shadow: var(--shadow-sm);
}

.error-message__text {
    margin: 0 0 16px;
    color: var(--error-text);
}

.error-message__button {
    padding: 8px 20px;
    font-size: 1rem;
    color: var(--primary-contrast);
    background-color: var(--primary);
    border: none;
    border-radius: var(--radius-sm);
    cursor: pointer;
}

.error-message__button:hover {
    opacity: 0.9;
}

button:focus-visible {
    outline: 2px solid var(--primary);
    outline-offset: 2px;
}

.element:focus-visible {
    outline: 2px solid var(--primary);
    outline-offset: 2px;
}

/* Modal chi tiết nguyên tố */
.element-dialog {
    position: fixed;
    inset: 0;
    margin: auto;
    width: min(720px, calc(100vw - 32px));
    max-width: calc(100vw - 32px);
    max-height: calc(100vh - 16px);
    max-height: calc(100dvh - 16px);
    padding: 0;
    border: 1px solid var(--card-border);
    border-radius: var(--radius-lg);
    background-color: var(--card-bg);
    color: var(--text);
    box-shadow: var(--shadow-lg);
    overscroll-behavior: contain;
}

.element-dialog::backdrop {
    background: rgba(15, 23, 42, 0.55);
    -webkit-backdrop-filter: blur(3px);
    backdrop-filter: blur(3px);
    animation: backdrop-in 200ms ease;
}

.element-dialog[open] {
    animation: dialog-in 220ms cubic-bezier(0.22, 0.61, 0.36, 1);
}

@keyframes dialog-in {
    from {
        opacity: 0;
        transform: translateY(14px) scale(0.96);
    }
    to {
        opacity: 1;
        transform: none;
    }
}

@keyframes backdrop-in {
    from {
        opacity: 0;
    }
    to {
        opacity: 1;
    }
}

.dialog__close {
    position: absolute;
    top: 12px;
    right: 12px;
    z-index: 3;
    display: grid;
    place-items: center;
    width: 36px;
    height: 36px;
    padding: 0;
    font-size: 1.1rem;
    line-height: 1;
    color: var(--text);
    background-color: var(--surface);
    border: 1px solid var(--border);
    border-radius: 50%;
    cursor: pointer;
    box-shadow: var(--shadow-sm);
    transition: border-color var(--transition-fast), transform var(--transition-fast), box-shadow var(--transition-fast);
}

.dialog__close:hover {
    border-color: var(--primary);
    transform: scale(1.06);
    box-shadow: var(--shadow-md);
}

.dialog__content {
    max-height: calc(100vh - 16px);
    max-height: calc(100dvh - 16px);
    overflow-y: auto;
    overscroll-behavior: contain;
    border-radius: var(--radius-lg);
}

.dialog__content::-webkit-scrollbar {
    width: 8px;
}

.dialog__content::-webkit-scrollbar-track {
    background: transparent;
}

.dialog__content::-webkit-scrollbar-thumb {
    background: var(--border);
    border-radius: 4px;
}

.element-header {
    display: flex;
    align-items: center;
    gap: 18px;
    padding: 20px 56px 20px 24px;
    background-color: color-mix(in srgb, var(--cat-bg, var(--card-bg)) 18%, var(--card-bg));
    border-bottom: 1px solid var(--border);
}

.element-header__info {
    min-width: 0;
}

.element-header__symbol {
    flex-shrink: 0;
    display: grid;
    place-items: center;
    width: 76px;
    height: 76px;
    font-size: 2.4rem;
    font-weight: 800;
    color: var(--cell-text);
    background-color: var(--cat-bg, var(--surface));
    border-radius: 18px;
    box-shadow: var(--shadow-md);
}

.element-header__name {
    margin: 0;
    font-size: 1.6rem;
    font-weight: 800;
    overflow-wrap: anywhere;
}

.element-header__name-row {
    display: flex;
    align-items: center;
    flex-wrap: wrap;
    gap: 10px;
    min-width: 0;
}

.speak-button {
    flex-shrink: 0;
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 44px;
    height: 44px;
    padding: 0;
    font-size: 1.15rem;
    line-height: 1;
    color: var(--text);
    background-color: var(--surface);
    border: 1px solid var(--border);
    border-radius: 50%;
    cursor: pointer;
    box-shadow: var(--shadow-xs);
    transition: border-color var(--transition-fast), transform var(--transition-fast), box-shadow var(--transition-fast), background-color var(--transition-fast);
}

.speak-button:hover {
    border-color: var(--primary);
    transform: translateY(-1px) scale(1.05);
    box-shadow: var(--shadow-sm);
    background-color: color-mix(in srgb, var(--surface) 85%, var(--primary) 15%);
}

.speak-button:active {
    transform: scale(0.92);
}

.element-header__number {
    margin: 4px 0 0;
    font-size: 0.9rem;
    color: var(--muted);
}

.element-media {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 16px;
    padding: 20px 24px;
    border-bottom: 1px solid var(--border);
}

.media-figure {
    margin: 0;
    display: flex;
    flex-direction: column;
    min-width: 0;
}

.media-figure__img,
.media-placeholder {
    width: 100%;
    height: 200px;
    object-fit: contain;
    padding: 8px;
    background-color: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-md);
}

.media-placeholder {
    display: grid;
    place-items: center;
    font-size: 3rem;
    font-weight: 800;
    color: var(--cell-text);
    background-color: var(--cat-bg, var(--surface));
    padding: 0;
}

.media-figure__caption {
    margin-top: 8px;
    font-size: 0.78rem;
    color: var(--muted);
}

.media-figure__attribution {
    display: block;
    margin-top: 2px;
    font-size: 0.7rem;
    opacity: 0.85;
    overflow-wrap: anywhere;
}

.media-figure__attribution-link {
    color: inherit;
    text-decoration: underline;
    text-underline-offset: 2px;
}

.media-figure__attribution-link:hover {
    color: var(--primary);
}

.dialog-section {
    padding: 18px 24px;
    border-bottom: 1px solid var(--border);
}

.dialog-section:last-of-type {
    border-bottom: none;
}

.dialog-section__title {
    margin: 0 0 10px;
    font-size: 0.8rem;
    font-weight: 700;
    letter-spacing: 0.05em;
    text-transform: uppercase;
    color: var(--muted);
}

.dialog-section__text {
    margin: 0;
    font-size: 0.92rem;
    line-height: 1.6;
}

.info-row {
    display: flex;
    justify-content: space-between;
    align-items: baseline;
    gap: 16px;
    padding: 7px 0;
}

.info-row + .info-row {
    border-top: 1px dashed var(--border);
}

.info-row--column {
    flex-direction: column;
    align-items: stretch;
    gap: 8px;
}

.info-row__label {
    flex-shrink: 0;
    font-size: 0.9rem;
    color: var(--muted);
}

.info-row__value {
    min-width: 0;
    font-size: 0.92rem;
    font-weight: 600;
    text-align: right;
    overflow-wrap: anywhere;
}

.info-row--column .info-row__value {
    text-align: left;
}

.ionization {
    display: flex;
    flex-wrap: wrap;
    gap: 6px;
}

.ionization__chip {
    padding: 3px 10px;
    font-size: 0.75rem;
    color: var(--text);
    background-color: var(--surface);
    border: 1px solid var(--border);
    border-radius: 999px;
}

.ionization__extra {
    display: contents;
}

.ionization__extra.is-hidden {
    display: none;
}

.ionization__toggle {
    width: max-content;
    margin-top: 4px;
    padding: 4px 12px;
    font-size: 0.78rem;
    color: var(--primary);
    background-color: transparent;
    border: 1px solid var(--border);
    border-radius: 999px;
    cursor: pointer;
    transition: border-color var(--transition-fast);
}

.ionization__toggle:hover {
    border-color: var(--primary);
}

.dialog-source {
    padding: 18px 24px 24px;
    text-align: center;
}

.dialog-source__link {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    padding: 10px 22px;
    font-size: 0.95rem;
    font-weight: 600;
    color: var(--primary-contrast);
    background-color: var(--primary);
    border-radius: 999px;
    text-decoration: none;
    box-shadow: var(--shadow-sm);
    transition: transform var(--transition-fast), box-shadow var(--transition-fast), filter var(--transition-fast);
}

.dialog-source__link:hover {
    transform: translateY(-1px);
    filter: brightness(1.08);
    box-shadow: var(--shadow-md);
}

@media (prefers-reduced-motion: reduce) {
    .element-dialog[open],
    .element-dialog::backdrop {
        animation: none;
    }
}

/* Toolbar Search / Filter */
.toolbar {
    display: flex;
    flex-wrap: wrap;
    align-items: center;
    gap: 10px;
    margin-top: 14px;
}

.toolbar__field {
    padding: 9px 14px;
    font-size: 0.9rem;
    font-family: inherit;
    color: var(--text);
    background-color: var(--surface);
    border: 1px solid var(--border);
    border-radius: 999px;
    box-shadow: var(--shadow-xs);
    outline: none;
    transition: border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.toolbar__search {
    flex: 1 1 220px;
    min-width: 160px;
}

.toolbar__field:focus-visible {
    border-color: var(--primary);
    box-shadow: 0 0 0 3px color-mix(in srgb, var(--primary) 25%, transparent);
}

select.toolbar__field {
    flex: 0 0 auto;
    cursor: pointer;
}

.toolbar__reset {
    padding: 9px 18px;
    font-size: 0.9rem;
    font-family: inherit;
    font-weight: 600;
    color: var(--text);
    background-color: var(--surface);
    border: 1px solid var(--border);
    border-radius: 999px;
    box-shadow: var(--shadow-xs);
    cursor: pointer;
    transition: border-color var(--transition-fast), color var(--transition-fast), transform var(--transition-fast);
}

.toolbar__reset:hover:not(:disabled) {
    border-color: var(--primary);
    color: var(--primary);
}

.toolbar__reset:disabled {
    opacity: 0.45;
    cursor: default;
}

.search-no-result {
    margin: 16px auto 0;
    max-width: 480px;
    text-align: center;
    font-size: 0.9rem;
    color: var(--muted);
}

/* Highlight / dim */
.element.is-dimmed {
    opacity: 0.25;
}

.element.is-dimmed:hover {
    opacity: 0.5;
}

.element.is-highlighted {
    border-color: var(--primary);
    box-shadow:
        0 0 0 2px var(--primary),
        0 0 18px color-mix(in srgb, var(--primary) 40%, transparent);
    z-index: 2;
}

.element.is-filter-match {
    border-color: color-mix(in srgb, var(--primary) 55%, var(--border));
}

/* ===== Học nhanh ===== */
.learn {
    margin-top: 32px;
    scroll-margin-top: 110px;
    padding: 24px;
    background-color: var(--card-bg);
    border: 1px solid var(--card-border);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-sm);
}

.learn__heading {
    margin-bottom: 18px;
}

.learn__title {
    margin: 0;
    font-size: 1.35rem;
    font-weight: 800;
}

.learn__subtitle {
    margin: 4px 0 0;
    font-size: 0.9rem;
    color: var(--muted);
}

.learn__grid {
    display: grid;
    grid-template-columns: repeat(2, minmax(0, 1fr));
    gap: 16px;
}

.learn__card {
    padding: 18px;
    background-color: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-md);
    box-shadow: var(--shadow-xs);
}

.learn__card--full {
    grid-column: 1 / -1;
}

.learn__card-title {
    margin: 0 0 12px;
    font-size: 1.02rem;
    font-weight: 800;
}

.learn__lead {
    margin: 0 0 12px;
    font-size: 0.9rem;
    color: var(--muted);
}

.learn__bullets {
    margin: 0;
    padding: 0;
    list-style: none;
    display: grid;
    gap: 10px;
    font-size: 0.92rem;
}

.learn__bullets li {
    display: flex;
    gap: 8px;
    align-items: flex-start;
}

.learn__bullets li::before {
    content: "•";
    flex-shrink: 0;
    color: var(--primary);
    font-weight: 800;
}

@media (min-width: 641px) {
    .learn__bullets--grid {
        grid-template-columns: repeat(2, minmax(0, 1fr));
        gap: 10px 16px;
    }
}

.learn__groups {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(min(230px, 100%), 1fr));
    gap: 12px;
}

.learn-group {
    padding: 12px;
    background-color: var(--card-bg);
    border: 1px solid var(--border);
    border-radius: var(--radius-sm);
}

.learn-group__title {
    display: flex;
    align-items: center;
    gap: 8px;
    margin: 0 0 4px;
    font-size: 0.92rem;
    font-weight: 700;
}

.learn-group__swatch {
    flex-shrink: 0;
    width: 12px;
    height: 12px;
    border-radius: 4px;
    background-color: var(--cat-bg, var(--surface));
    border: 1px solid rgba(15, 23, 42, 0.18);
}

.learn-group__desc {
    margin: 0 0 10px;
    font-size: 0.8rem;
    line-height: 1.45;
    color: var(--muted);
}

.learn-group__chips {
    display: flex;
    flex-wrap: wrap;
    gap: 6px;
}

.learn-chip {
    display: inline-flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    gap: 2px;
    min-height: 44px;
    min-width: 44px;
    padding: 6px 10px;
    font-family: inherit;
    color: var(--cell-text);
    background-color: var(--cat-bg, var(--surface));
    border: 1px solid rgba(15, 23, 42, 0.12);
    border-radius: var(--radius-sm);
    cursor: pointer;
    box-shadow: var(--shadow-xs);
    transition: transform var(--transition-fast), border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.learn-chip:hover,
.learn-chip:focus-visible {
    transform: translateY(-2px);
    border-color: var(--primary);
    box-shadow: var(--shadow-sm);
    outline: none;
}

.learn-chip__symbol {
    font-size: 0.92rem;
    font-weight: 800;
    line-height: 1;
}

.learn-chip__number {
    font-size: 0.66rem;
    color: var(--cell-muted);
    line-height: 1;
}

.learn__common {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(min(96px, 100%), 1fr));
    gap: 8px;
}

.learn-chip--common {
    min-height: 58px;
    padding: 8px 6px;
}

.learn-chip--common .learn-chip__name {
    font-size: 0.72rem;
    color: var(--cell-muted);
    max-width: 100%;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
}

.learn__steps {
    margin: 0 0 14px;
    padding: 0;
    list-style: none;
    display: grid;
    gap: 10px;
}

@media (min-width: 641px) {
    .learn__steps {
        grid-template-columns: repeat(2, minmax(0, 1fr));
    }
}

.learn__steps li {
    display: flex;
    align-items: flex-start;
    gap: 10px;
    padding: 10px 12px;
    font-size: 0.9rem;
    background-color: var(--card-bg);
    border: 1px solid var(--border);
    border-radius: var(--radius-sm);
}

.learn__step-num {
    flex-shrink: 0;
    display: grid;
    place-items: center;
    width: 26px;
    height: 26px;
    font-size: 0.85rem;
    font-weight: 700;
    color: var(--primary-contrast);
    background: linear-gradient(135deg, var(--primary), var(--accent));
    border-radius: 50%;
}

.learn__advice {
    margin: 0;
    padding: 12px 14px;
    font-size: 0.9rem;
    background-color: color-mix(in srgb, var(--primary) 10%, var(--surface));
    border: 1px solid color-mix(in srgb, var(--primary) 32%, var(--border));
    border-radius: var(--radius-md);
}

/* Tablet (641–1024) tweaks */
@media (min-width: 641px) and (max-width: 1024px) {
    .learn {
        scroll-margin-top: 96px;
    }
}

/* Mobile */
@media (max-width: 640px) {
    :root {
        --cell-size: 58px;
        --gap: 3px;
    }

    .container {
        padding-inline: 12px;
    }

    .header {
        padding: 10px 0;
    }

    .header__inner {
        gap: 10px;
    }

    .header__title {
        font-size: 1.15rem;
    }

    .header__subtitle {
        font-size: 0.75rem;
    }

    .theme-toggle {
        width: 40px;
        height: 40px;
        font-size: 1.15rem;
    }

    .learn-link {
        padding: 8px 12px;
        font-size: 0.8rem;
    }

    .toolbar {
        align-items: stretch;
        gap: 8px;
        margin-top: 10px;
    }

    .toolbar__search {
        flex-basis: 100%;
        min-width: 0;
    }

    .toolbar__field,
    .toolbar__reset {
        min-height: 46px;
        padding: 9px 10px;
        font-size: 16px;
    }

    select.toolbar__field,
    .toolbar__reset {
        flex: 1 1 0;
        min-width: 0;
    }

    .main {
        padding: 18px 0 32px;
    }

    .periodic-view.is-table {
        padding: 12px;
    }

    .explorer {
        gap: 16px;
    }

    .explorer-group__grid {
        grid-template-columns: repeat(2, minmax(0, 1fr));
        gap: 8px;
    }

    .element--card {
        min-height: 84px;
    }

    .legend {
        gap: 6px;
        padding: 10px;
    }

    .legend__chip {
        gap: 5px;
        padding: 4px 9px;
        font-size: 0.72rem;
    }

    .legend__swatch {
        width: 10px;
        height: 10px;
    }

    .series-note {
        gap: 6px;
        margin-top: 14px;
    }

    .series-note__item {
        gap: 5px;
        padding: 4px 10px;
        font-size: 0.72rem;
    }

    .series-note__swatch {
        width: 10px;
        height: 10px;
    }

    /* Modal near fullscreen */
    .element-dialog {
        width: calc(100vw - 16px);
        max-width: calc(100vw - 16px);
        max-height: calc(100vh - 16px);
        max-height: calc(100dvh - 16px);
        margin: auto;
    }

    .dialog__close {
        top: 10px;
        right: 10px;
        width: 44px;
        height: 44px;
        font-size: 1.2rem;
    }

    .element-header {
        flex-wrap: wrap;
        gap: 12px;
        padding: 16px 52px 14px 16px;
    }

    .element-header__symbol {
        width: 58px;
        height: 58px;
        font-size: 1.8rem;
        border-radius: 14px;
    }

    .element-header__name {
        font-size: 1.25rem;
    }

    .element-header__number {
        font-size: 0.82rem;
    }

    .element-media {
        grid-template-columns: 1fr;
        padding: 14px 16px;
        gap: 12px;
    }

    .media-figure__img,
    .media-placeholder {
        height: 170px;
    }

    .dialog-section {
        padding: 14px 16px;
    }

    .dialog-source {
        padding: 14px 16px 18px;
    }

    .dialog__content {
        max-height: calc(100vh - 16px);
        max-height: calc(100dvh - 16px);
    }

    /* Học nhanh */
    .learn {
        margin-top: 24px;
        padding: 16px 14px;
        scroll-margin-top: 96px;
    }

    .learn__grid {
        grid-template-columns: 1fr;
    }

    .learn__card {
        padding: 14px;
    }

    .learn__card--full {
        grid-column: auto;
    }

    .learn__bullets--grid {
        grid-template-columns: 1fr;
    }

    .learn__groups {
        grid-template-columns: 1fr;
    }

    .learn__steps {
        grid-template-columns: 1fr;
    }
}

@media (max-width: 480px) {
    .header__subtitle {
        display: none;
    }

    .header__inner {
        gap: 10px;
    }

    .header__actions {
        gap: 8px;
    }

    .header__title {
        font-size: 0.92rem;
    }

    .learn-link {
        padding: 7px 10px;
        font-size: 0.75rem;
    }

    .theme-toggle {
        width: 38px;
        height: 38px;
        font-size: 1.05rem;
    }
}