:root {
    color-scheme: light;
    --ink: #18212a;
    --muted: #5d6874;
    --line: #d9dde3;
    --panel: #ffffff;
    --surface: #f5f7f2;
    --accent: #b65b3b;
    --accent-dark: #88402a;
    --steel: #2f5967;
    --gold: #d8a84e;
}

* {
    box-sizing: border-box;
}

body {
    margin: 0;
    min-width: 320px;
    background: var(--surface);
    color: var(--ink);
    font-family: Arial, Helvetica, sans-serif;
}

button,
input {
    font: inherit;
}

.site-return {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 18px;
    min-height: 62px;
    padding: 12px clamp(18px, 4vw, 44px);
    background: #0b0d11;
    border-bottom: 1px solid rgb(255 255 255 / 10%);
    color: #ffffff;
}

.site-return a {
    color: inherit;
    text-decoration: none;
}

.site-return-brand {
    display: grid;
    gap: 3px;
}

.site-return-brand strong {
    font-size: 1rem;
    letter-spacing: 0;
}

.site-return-brand span {
    color: #b8c2cf;
    font-size: 0.86rem;
}

.site-return-link {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    min-height: 38px;
    padding: 8px 14px;
    border: 1px solid rgb(255 255 255 / 18%);
    border-radius: 999px;
    background: #159dcc;
    color: #071014;
    font-weight: 800;
}

.site-return-link:hover {
    background: #20b5e8;
}

.site-return-link svg {
    width: 18px;
    height: 18px;
    fill: currentColor;
}

.app-shell {
    display: grid;
    grid-template-columns: 310px minmax(0, 1fr);
    min-height: calc(100vh - 62px);
}

.sidebar {
    background: #18252c;
    color: #f8fbfc;
    padding: 22px;
    overflow-y: auto;
}

.brand {
    display: flex;
    align-items: center;
    gap: 12px;
    padding-bottom: 22px;
    border-bottom: 1px solid rgb(255 255 255 / 18%);
}

.brand-mark {
    display: grid;
    place-items: center;
    width: 42px;
    height: 42px;
    border-radius: 8px;
    background: var(--gold);
    color: #1f2528;
    font-weight: 800;
}

.brand strong,
.brand span {
    display: block;
}

.brand span {
    color: #bfccd0;
    font-size: 0.86rem;
    margin-top: 3px;
}

.course-nav {
    display: grid;
    gap: 8px;
    padding-top: 22px;
}

.nav-group {
    border-radius: 6px;
}

.nav-group summary {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 10px;
    min-height: 34px;
    padding: 8px 6px 8px 0;
    color: #dbe5e8;
    font-size: 0.82rem;
    font-weight: 700;
    text-transform: uppercase;
    cursor: pointer;
    list-style: none;
}

.nav-group summary::-webkit-details-marker {
    display: none;
}

.nav-group summary::after {
    content: "";
    width: 8px;
    height: 8px;
    border-right: 2px solid #bfccd0;
    border-bottom: 2px solid #bfccd0;
    transform: rotate(-45deg);
    transition: transform 0.16s ease;
}

.nav-group[open] summary::after {
    transform: rotate(45deg);
}

.nav-group summary:hover {
    color: #ffffff;
}

.nav-subcategories {
    display: grid;
    gap: 4px;
    padding: 2px 0 10px;
}

.nav-group a {
    display: block;
    min-height: 36px;
    padding: 9px 10px;
    border-radius: 6px;
    color: #e9f0f2;
    text-decoration: none;
}

.nav-group a:hover,
.nav-group a.active {
    background: rgb(216 168 78 / 18%);
    color: #ffffff;
}

.workspace {
    min-width: 0;
}

.topbar {
    position: sticky;
    top: 0;
    z-index: 3;
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 20px;
    padding: 28px 34px 20px;
    background: rgb(245 247 242 / 94%);
    border-bottom: 1px solid var(--line);
    backdrop-filter: blur(10px);
}

.eyebrow {
    color: var(--steel);
    font-size: 0.8rem;
    font-weight: 700;
    text-transform: uppercase;
}

h1,
h2,
h3,
p {
    overflow-wrap: anywhere;
}

h1 {
    margin: 6px 0 0;
    font-size: clamp(1.65rem, 2.4vw, 2.35rem);
    letter-spacing: 0;
}

.primary-action,
.secondary-action,
.tool-panel button,
.calculator-switcher button,
.related-calculators button {
    min-height: 40px;
    border: 1px solid transparent;
    border-radius: 6px;
    cursor: pointer;
}

.topbar-actions {
    display: flex;
    gap: 10px;
}

.primary-action {
    padding: 10px 18px;
    background: var(--accent);
    color: #ffffff;
    font-weight: 700;
}

.primary-action.compact {
    min-height: 34px;
    padding: 6px 12px;
}

.primary-action:hover {
    background: var(--accent-dark);
}

.secondary-action {
    padding: 10px 16px;
    background: #ffffff;
    border-color: var(--line);
    color: var(--ink);
    font-weight: 700;
}

.secondary-action:hover {
    border-color: var(--steel);
}

.content-grid {
    display: grid;
    grid-template-columns: minmax(0, 1fr) 300px;
    gap: 24px;
    width: min(1260px, 100%);
    margin: 0 auto;
    padding: 28px 34px 44px;
}

.editor-toolbar {
    grid-column: 1 / -1;
    display: flex;
    align-items: center;
    flex-wrap: wrap;
    gap: 10px;
    padding: 10px 12px;
    border: 1px solid var(--line);
    border-radius: 8px;
    background: linear-gradient(#ffffff, #f8faf9);
    box-shadow: 0 1px 2px rgb(24 33 42 / 6%);
}

.editor-toolbar[hidden] {
    display: none;
}

.toolbar-group {
    display: inline-flex;
    align-items: center;
    gap: 2px;
    padding: 3px;
    border: 1px solid #d8e0e2;
    border-radius: 7px;
    background: #eef4f4;
}

.editor-toolbar button {
    border: 1px solid transparent;
    border-radius: 5px;
    color: var(--ink);
    cursor: pointer;
}

.editor-toolbar .format-button {
    display: inline-grid;
    place-items: center;
    width: 34px;
    height: 32px;
    padding: 0;
    background: transparent;
}

.format-button svg {
    width: 19px;
    height: 19px;
    fill: currentColor;
}

.editor-toolbar .format-button:hover,
.editor-toolbar .format-button:focus-visible {
    border-color: #b8c8cc;
    background: #ffffff;
    color: var(--steel);
    outline: none;
}

.editor-toolbar .format-button:active {
    background: #dfeaea;
    transform: translateY(1px);
}

.save-button {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    width: auto;
    margin-left: 2px;
}

.save-button svg {
    width: 17px;
    height: 17px;
    fill: currentColor;
}

.save-status {
    color: var(--muted);
    font-size: 0.9rem;
    font-weight: 700;
}

.lesson,
.tool-panel,
.calculator-modal {
    border: 1px solid var(--line);
    border-radius: 8px;
    background: var(--panel);
}

.lesson {
    padding: 30px;
    line-height: 1.65;
}

.lesson[contenteditable="true"] {
    outline: 3px solid rgb(47 89 103 / 22%);
    outline-offset: 3px;
}

.lesson p,
.lesson li {
    margin: 0 0 14px;
}

.lesson img {
    display: block;
    max-width: 100%;
    height: auto;
    margin: 18px auto;
    border: 1px solid var(--line);
    border-radius: 6px;
}

.lesson li {
    margin-left: 18px;
}

.tool-panel {
    align-self: start;
    position: sticky;
    top: 112px;
    padding: 18px;
}

.tool-panel h2 {
    margin: 0 0 14px;
    font-size: 1rem;
}

.tool-panel button {
    display: grid;
    width: 100%;
    gap: 4px;
    margin-bottom: 10px;
    padding: 11px;
    background: #eef3f3;
    color: var(--ink);
    text-align: left;
}

.tool-panel button:hover {
    border-color: var(--steel);
}

.tool-panel small {
    color: var(--muted);
}

.calculator-modal {
    width: min(920px, calc(100vw - 28px));
    padding: 0;
    box-shadow: 0 24px 80px rgb(0 0 0 / 28%);
}

.calculator-modal::backdrop {
    background: rgb(20 28 32 / 64%);
}

.modal-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 16px;
    padding: 20px 22px;
    border-bottom: 1px solid var(--line);
}

.modal-header h2 {
    margin: 4px 0 0;
    font-size: 1.35rem;
}

.icon-button {
    width: 40px;
    height: 40px;
    border: 1px solid var(--line);
    border-radius: 6px;
    background: #ffffff;
    cursor: pointer;
    font-size: 1.4rem;
}

.modal-body {
    padding: 20px 22px 24px;
}

.calculator-switcher {
    display: flex;
    gap: 8px;
    margin-bottom: 18px;
    overflow-x: auto;
}

.calculator-switcher button,
.related-calculators button {
    flex: 0 0 auto;
    padding: 8px 11px;
    background: #eef3f3;
    color: var(--ink);
}

.calculator-switcher button.active,
.related-calculators button:hover {
    background: var(--steel);
    color: #ffffff;
}

.calculator-layout {
    display: grid;
    grid-template-columns: minmax(0, 1fr) 280px;
    gap: 20px;
}

.formula {
    margin: 0 0 16px;
    padding: 12px;
    border-left: 4px solid var(--gold);
    background: #fff9e8;
    font-weight: 700;
}

.field {
    display: grid;
    gap: 7px;
    margin-bottom: 14px;
}

.field label {
    color: var(--muted);
    font-size: 0.9rem;
    font-weight: 700;
}

.field input {
    width: 100%;
    min-height: 42px;
    padding: 9px 11px;
    border: 1px solid var(--line);
    border-radius: 6px;
}

.field small {
    color: var(--muted);
}

.result-panel {
    padding: 16px;
    border-radius: 8px;
    background: #f6f8f8;
}

.result-panel h3 {
    margin: 0 0 8px;
    font-size: 0.92rem;
}

.result-panel output {
    display: block;
    min-height: 44px;
    margin-bottom: 18px;
    color: var(--accent-dark);
    font-size: 1.3rem;
    font-weight: 800;
}

.result-panel ul {
    margin: 0 0 18px;
    padding-left: 18px;
}

.related-calculators {
    display: flex;
    flex-wrap: wrap;
    gap: 7px;
}

@media (max-width: 980px) {
    .app-shell,
    .content-grid,
    .calculator-layout {
        grid-template-columns: 1fr;
    }

    .sidebar {
        max-height: 44vh;
    }

    .topbar,
    .content-grid {
        padding-left: 18px;
        padding-right: 18px;
    }

    .tool-panel {
        position: static;
    }
}

@media (max-width: 620px) {
    .site-return {
        align-items: flex-start;
        flex-direction: column;
    }

    .site-return-link {
        width: 100%;
        justify-content: center;
    }

    .topbar {
        align-items: flex-start;
        flex-direction: column;
    }

    .topbar-actions {
        width: 100%;
    }

    .topbar-actions button {
        flex: 1;
    }

    .primary-action {
        width: 100%;
    }

    .lesson {
        padding: 20px;
    }
}