:root {
    --bg-primary: rgb(245, 247, 250);
    --bg-secondary: rgb(255, 255, 255);
    --text-main: rgb(44, 62, 80);
    --text-muted: rgb(127, 140, 141);
    --accent: rgb(29, 158, 55);
    --accent-hover: rgb(83, 219, 52);
    --correct: rgb(39, 174, 96);
    --incorrect: rgb(192, 57, 43);
    --border: rgb(223, 230, 233);
}

body.dark-mode {
    --bg-primary: rgb(15, 15, 15);
    --bg-secondary: rgb(25, 25, 25);
    --text-main: rgb(220, 220, 220);
    --text-muted: rgb(150, 150, 150);
    --accent: rgb(29, 158, 55);
    --accent-hover: rgb(83, 219, 52);
    --border: rgb(45, 45, 45);
}

body {
    margin: 0;
    font-family: 'Tahoma', Geneva, Verdana, sans-serif;
    background-color: var(--bg-primary);
    color: var(--text-main);
    transition: background-color 0.3s, color 0.3s;
    line-height: 1.8;
}

.hidden {
    display: none !important;
}

.center-text {
    text-align: center;
}

.navbar {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 1.5rem 2rem;
    background-color: var(--bg-secondary);
    border-bottom: 1px solid var(--border);
    box-shadow: rgb(0, 0, 0, 0.05) 0px 2px 5px;
}

.logo {
    font-family: 'Space Grotesk', sans-serif;
    font-size: 1.5rem;
    font-weight: 700;
    color: var(--accent);
}

.nav-controls {
    display: flex;
    gap: 1rem;
}

.btn {
    padding: 0.6rem 1.2rem;
    border: none;
    border-radius: 6px;
    cursor: pointer;
    font-weight: 600;
    font-size: 1rem;
    transition: all 0.2s;
    background-color: var(--accent);
    color: rgb(255, 255, 255);
}

.btn:hover {
    background-color: var(--accent-hover);
}

.hero {
    position: relative;
    height: 380px;
    display: flex;
    align-items: center;
    justify-content: center;
    overflow: hidden;
    background: linear-gradient(135deg, rgb(129, 137, 155) 0%);
}

.hero-content {
    position: relative;
    z-index: 10;
    text-align: center;
    color: rgb(255, 255, 255);
    max-width: 900px;
    padding: 2.5rem;
    background-color: rgba(0, 0, 0, 0.7);
    border-radius: 12px;
}

.hero-content h1 {
    font-size: 2.2rem;
    margin-bottom: 1rem;
}

.main-container {
    max-width: 1200px;
    margin: 3rem auto;
    padding: 0 1.5rem;
}

.subjects-grid {
    display: flex;
    gap: 2rem;
    margin-top: 3rem;
    justify-content: center;
}

.card {
    background-color: var(--bg-secondary);
    border: 1px solid var(--border);
    padding: 3rem 2rem;
    border-radius: 12px;
    flex: 1;
    text-align: center;
    cursor: pointer;
    transition: all 0.3s ease;
}

.card:hover {
    transform: translateY(-8px);
    border-color: var(--accent);
}

.topic-list {
    display: grid;
    grid-template-columns: 1fr;
    gap: 2rem;
    margin-top: 2rem;
}

.topic-item {
    background-color: var(--bg-secondary);
    padding: 2rem;
    border: 1px solid var(--border);
    border-radius: 12px;
}

.topic-title {
    color: var(--accent);
    font-size: 1.5rem;
    margin-top: 0;
    margin-bottom: 0.5rem;
}

.subtopic-list {
    margin-top: 1.5rem;
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 1rem;
}

.subtopic-item {
    padding: 1.2rem;
    background-color: var(--bg-primary);
    cursor: pointer;
    border-radius: 8px;
    border-left: 5px solid var(--accent);
    font-weight: 600;
}

.subtopic-item:hover {
    background-color: var(--border);
}

.lesson-box {
    background-color: var(--bg-secondary);
    padding: 3rem;
    border-radius: 12px;
    margin-top: 2rem;
    border: 1px solid var(--border);
    font-size: 1.1rem;
}

.lesson-box p {
    margin-bottom: 1.5rem;
}

.lesson-title {
    font-size: 2rem;
    color: var(--accent);
    border-bottom: 2px solid var(--border);
    padding-bottom: 1rem;
    margin-bottom: 2rem;
}

.problem-container {
    margin-top: 2.5rem;
    padding: 2rem;
    border: 2px solid var(--border);
    border-radius: 12px;
    background-color: var(--bg-secondary);
}

.problem-text {
    font-size: 1.2rem;
    font-weight: 600;
    margin-bottom: 1.5rem;
}

.options-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 1.2rem;
}

.option-btn {
    background-color: var(--bg-primary);
    color: var(--text-main);
    border: 1px solid var(--border);
    padding: 1.2rem;
    border-radius: 8px;
    cursor: pointer;
    font-size: 1.1rem;
}

.option-btn:hover:not(:disabled) {
    background-color: var(--border);
}

.option-btn:disabled {
    cursor: not-allowed;
    opacity: 0.7;
}

.option-btn.correct-ans {
    background-color: var(--correct);
    color: rgb(255, 255, 255);
    border-color: var(--correct);
}

.option-btn.incorrect-ans {
    background-color: var(--incorrect);
    color: rgb(255, 255, 255);
    border-color: var(--incorrect);
}

.feedback {
    margin-top: 1.5rem;
    font-weight: bold;
    font-size: 1.2rem;
    text-align: center;
    min-height: 1.5em;
}