:root {
	--bg: #eef3fb;
	--bg-strong: #dfe8f7;
	--panel: rgba(252, 254, 255, 0.9);
	--panel-strong: #fcfeff;
	--border: rgba(20, 47, 87, 0.13);
	--text: #132032;
	--muted: #5b6a7d;
	--accent: #1f5fbf;
	--accent-soft: #dbe9ff;
	--accent-strong: #123d7f;
	--success: #0e7457;
	--warning: #8e5f1a;
	--danger: #a23838;
	--shadow: 0 20px 52px rgba(20, 47, 87, 0.12);
	--radius: 22px;
	--mono: "IBM Plex Mono", "SFMono-Regular", Consolas, monospace;
	--serif: "Iowan Old Style", "Palatino Linotype", "Book Antiqua", Georgia,
		serif;
	--sans: "Avenir Next", "Segoe UI", system-ui, sans-serif;
}

* {
	box-sizing: border-box;
}

html {
	scroll-behavior: smooth;
}

body {
	margin: 0;
	font-family: var(--sans);
	color: var(--text);
	background:
		radial-gradient(circle at top left, rgba(179, 212, 255, 0.8), transparent 28%),
		radial-gradient(circle at top right, rgba(223, 232, 247, 0.9), transparent 24%),
		linear-gradient(180deg, #f8fbff 0%, var(--bg) 46%, #e6edf8 100%);
	min-height: 100vh;
}

a {
	color: var(--accent-strong);
}

.page-shell {
	display: grid;
	grid-template-columns: minmax(290px, 340px) minmax(0, 1fr);
	gap: 28px;
	max-width: 1500px;
	margin: 0 auto;
	padding: 26px;
}

.sidebar {
	position: sticky;
	top: 0;
	align-self: start;
	padding: 28px;
	border-radius: calc(var(--radius) + 8px);
	background:
		linear-gradient(180deg, rgba(252, 254, 255, 0.96), rgba(243, 248, 255, 0.88)),
		var(--panel);
	border: 1px solid var(--border);
	box-shadow: var(--shadow);
	backdrop-filter: blur(14px);
	max-height: calc(100vh - 52px);
	overflow: auto;
}

.sidebar h1,
.content h2,
.content h3,
.content h4,
.hub-shell h1,
.hub-shell h2,
.hub-shell h3 {
	font-family: var(--serif);
	letter-spacing: -0.02em;
	margin: 0 0 12px;
}

.sidebar h1,
.hub-shell h1 {
	font-size: clamp(2.3rem, 4vw, 3.4rem);
	line-height: 0.95;
}

.sidebar-copy,
.card p,
.explanation-box,
.quiz-result,
.callout p,
.clean-list li,
.metric p,
.course-card p {
	line-height: 1.66;
	font-size: 1rem;
}

.eyebrow {
	margin: 0 0 10px;
	text-transform: uppercase;
	letter-spacing: 0.18em;
	font-size: 0.78rem;
	color: var(--muted);
}

.panel,
.lesson,
.card,
.callout,
.course-card,
.metric {
	border: 1px solid var(--border);
	box-shadow: var(--shadow);
}

.panel {
	background: rgba(252, 254, 255, 0.9);
	border-radius: 20px;
	padding: 18px;
	margin-top: 22px;
}

.panel-head,
.card-head,
.lesson-top {
	display: flex;
	align-items: center;
	justify-content: space-between;
	gap: 12px;
}

.nav-panel {
	display: grid;
	gap: 10px;
}

.nav-panel a,
.back-link {
	padding: 10px 12px;
	border-radius: 12px;
	background: rgba(244, 248, 255, 0.88);
	text-decoration: none;
	font-weight: 600;
	display: inline-block;
	transition:
		transform 0.16s ease,
		background 0.16s ease;
}

.nav-panel a:hover,
.nav-panel a:focus-visible,
.back-link:hover,
.back-link:focus-visible {
	background: var(--accent-soft);
	transform: translateX(4px);
}

.progress-track {
	height: 13px;
	background: rgba(91, 106, 125, 0.14);
	border-radius: 999px;
	overflow: hidden;
	margin: 12px 0 14px;
}

.progress-fill {
	height: 100%;
	width: 0;
	background: linear-gradient(90deg, var(--accent), #57a0ff);
	border-radius: inherit;
	transition: width 0.25s ease;
}

button,
.ghost-button {
	border: 1px solid rgba(18, 61, 127, 0.18);
	border-radius: 999px;
	background: rgba(252, 254, 255, 0.96);
	color: var(--text);
	font: inherit;
	padding: 10px 14px;
	cursor: pointer;
	transition:
		transform 0.16s ease,
		background 0.16s ease,
		border-color 0.16s ease;
}

button:hover,
button:focus-visible,
.ghost-button:hover,
.ghost-button:focus-visible {
	transform: translateY(-1px);
	border-color: rgba(18, 61, 127, 0.38);
	background: rgba(233, 243, 255, 1);
}

.content {
	display: grid;
	gap: 24px;
}

.lesson,
.extra-lesson {
	padding: 26px;
	background:
		linear-gradient(180deg, rgba(252, 254, 255, 0.97), rgba(245, 249, 255, 0.93)),
		var(--panel-strong);
	border-radius: calc(var(--radius) + 2px);
}

.lesson.is-complete {
	border-color: rgba(14, 116, 87, 0.32);
	box-shadow:
		var(--shadow),
		inset 0 0 0 2px rgba(14, 116, 87, 0.08);
}

.lesson-check {
	display: inline-flex;
	align-items: center;
	gap: 10px;
	font-weight: 600;
}

.lesson-check input {
	inline-size: 18px;
	block-size: 18px;
	accent-color: var(--success);
}

.two-column,
.three-column,
.four-grid,
.interactive-grid,
.course-grid,
.metric-grid,
.flashcard-grid {
	display: grid;
	gap: 18px;
}

.two-column {
	grid-template-columns: repeat(2, minmax(0, 1fr));
}

.three-column {
	grid-template-columns: repeat(3, minmax(0, 1fr));
}

.four-grid {
	grid-template-columns: repeat(4, minmax(0, 1fr));
}

.interactive-grid {
	grid-template-columns: minmax(0, 1.05fr) minmax(0, 0.95fr);
	align-items: start;
}

.course-grid {
	grid-template-columns: repeat(3, minmax(0, 1fr));
}

.metric-grid {
	grid-template-columns: repeat(4, minmax(0, 1fr));
}

.card,
.course-card,
.metric {
	padding: 20px;
	border-radius: 18px;
	background: rgba(252, 254, 255, 0.92);
}

.course-card,
.metric {
	text-decoration: none;
	color: inherit;
}

.course-card:hover,
.course-card:focus-visible {
	transform: translateY(-3px);
	background: rgba(248, 252, 255, 0.98);
}

.course-card h3,
.metric h3 {
	margin-bottom: 8px;
}

.callout {
	margin-top: 18px;
	padding: 18px 20px;
	border-radius: 18px;
	background: rgba(235, 244, 255, 0.95);
}

.callout.warning {
	background: rgba(255, 245, 224, 0.95);
	border-color: rgba(142, 95, 26, 0.26);
}

.callout.danger {
	background: rgba(255, 236, 236, 0.95);
	border-color: rgba(162, 56, 56, 0.26);
}

.badge-row {
	display: flex;
	flex-wrap: wrap;
	gap: 10px;
	margin-top: 14px;
}

.badge {
	padding: 8px 12px;
	border-radius: 999px;
	background: var(--accent-soft);
	color: var(--accent-strong);
	font-weight: 700;
	font-size: 0.92rem;
}

.code-block,
.diagram {
	padding: 16px 18px;
	border-radius: 16px;
	font-family: var(--mono);
	font-size: 0.92rem;
	line-height: 1.62;
	white-space: pre-wrap;
	overflow-x: auto;
	border: 1px solid rgba(20, 47, 87, 0.12);
}

.code-block {
	background: #11233b;
	color: #f4f9ff;
}

.diagram {
	background: #e7eef9;
	color: #102135;
}

.clean-list {
	margin: 0;
	padding-left: 20px;
}

.button-row {
	display: flex;
	flex-wrap: wrap;
	gap: 10px;
}

.switch-button.is-active {
	background: var(--accent-soft);
	border-color: rgba(18, 61, 127, 0.42);
	color: var(--accent-strong);
}

.explanation-box,
.step-card,
.quiz-result,
.table-box {
	padding: 18px;
	border-radius: 16px;
	background: rgba(247, 251, 255, 0.96);
	border: 1px solid rgba(20, 47, 87, 0.1);
	line-height: 1.66;
}

.step-count {
	margin: 0 0 8px;
	color: var(--muted);
	font-weight: 700;
}

.table-box table {
	width: 100%;
	border-collapse: collapse;
}

.table-box th,
.table-box td {
	text-align: left;
	padding: 10px 12px;
	border-bottom: 1px solid rgba(20, 47, 87, 0.1);
	vertical-align: top;
}

.table-box th {
	font-family: var(--serif);
}

.flashcard-grid {
	grid-template-columns: repeat(4, minmax(0, 1fr));
}

.flashcard {
	position: relative;
	min-height: 180px;
	padding: 0;
	border-radius: 18px;
	overflow: hidden;
	perspective: 1000px;
	background: transparent;
	border: none;
}

.flashcard .face {
	position: absolute;
	inset: 0;
	display: grid;
	place-items: center;
	text-align: center;
	padding: 18px;
	border-radius: 18px;
	backface-visibility: hidden;
	transition: transform 0.4s ease;
	box-shadow: var(--shadow);
}

.flashcard .front {
	background: linear-gradient(180deg, #f8fbff, #deebff);
	font-family: var(--serif);
	font-size: 1.3rem;
}

.flashcard .back {
	background: linear-gradient(180deg, #15273e, #0d1828);
	color: #f2f8ff;
	transform: rotateY(180deg);
	line-height: 1.6;
}

.flashcard.is-flipped .front {
	transform: rotateY(180deg);
}

.flashcard.is-flipped .back {
	transform: rotateY(360deg);
}

.quiz-form {
	display: grid;
	gap: 18px;
	margin-top: 12px;
}

.quiz-question {
	padding: 16px;
	border-radius: 16px;
	background: rgba(252, 254, 255, 0.97);
	border: 1px solid rgba(20, 47, 87, 0.1);
}

.quiz-question legend {
	font-weight: 700;
	margin-bottom: 12px;
}

.quiz-option {
	display: flex;
	align-items: flex-start;
	gap: 10px;
	padding: 8px 0;
}

.quiz-option input {
	margin-top: 4px;
	accent-color: var(--accent);
}

.quiz-result.is-good {
	border-color: rgba(14, 116, 87, 0.34);
	background: rgba(228, 248, 240, 0.92);
}

.quiz-result.is-mid {
	border-color: rgba(142, 95, 26, 0.34);
	background: rgba(255, 245, 224, 0.95);
}

.quiz-result.is-bad {
	border-color: rgba(162, 56, 56, 0.34);
	background: rgba(255, 236, 236, 0.95);
}

.exercise + .exercise {
	margin-top: 16px;
}

.hidden {
	display: none;
}

.hub-shell {
	max-width: 1380px;
	margin: 0 auto;
	padding: 28px;
}

.hero,
.hub-section {
	padding: 28px;
	border-radius: calc(var(--radius) + 4px);
	background:
		linear-gradient(180deg, rgba(252, 254, 255, 0.97), rgba(244, 249, 255, 0.93)),
		var(--panel);
	border: 1px solid var(--border);
	box-shadow: var(--shadow);
}

.hero {
	display: grid;
	grid-template-columns: minmax(0, 1.2fr) minmax(300px, 0.8fr);
	gap: 24px;
	align-items: center;
}

.hero-copy {
	font-size: 1.08rem;
	line-height: 1.72;
	max-width: 65ch;
}

.hero-ascii {
	margin: 0;
}

.hub-stack {
	display: grid;
	gap: 24px;
	margin-top: 24px;
}

@media (max-width: 1100px) {
	.page-shell,
	.hero {
		grid-template-columns: 1fr;
	}

	.sidebar {
		position: static;
		max-height: none;
	}

	.two-column,
	.three-column,
	.four-grid,
	.interactive-grid,
	.course-grid,
	.metric-grid,
	.flashcard-grid {
		grid-template-columns: 1fr;
	}
}

@media (max-width: 720px) {
	.page-shell,
	.hub-shell {
		padding: 16px;
		gap: 18px;
	}

	.sidebar,
	.lesson,
	.hero,
	.hub-section {
		padding: 18px;
	}

	.lesson-top,
	.card-head,
	.panel-head {
		flex-direction: column;
		align-items: flex-start;
	}

	.button-row {
		width: 100%;
	}

	button {
		width: 100%;
	}
}