body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
    background-color: #f3f4f6;
    color: #1f2937;
    margin: 0;
    padding: 24px;
}
.container {
    max-width: 760px;
    margin: 0 auto;
}
header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 24px;
}
.brand h1 {
    font-size: 1.6rem;
    margin: 0;
    color: #111827;
    letter-spacing: -0.02em;
}
.tagline {
    font-size: 0.85rem;
    color: #6b7280;
}
.header-controls {
    display: flex;
    align-items: center;
    gap: 12px;
}
.mastery-badge {
    background-color: #e5e7eb;
    color: #374151;
    padding: 6px 14px;
    border-radius: 9999px;
    font-weight: 600;
    font-size: 0.95rem;
    transition: all 0.2s ease;
}
.mastery-badge.mastered {
    background-color: #dcfce7;
    color: #15803d;
    border: 1px solid #86efac;
}
.text-btn {
    background: none;
    border: none;
    color: #9ca3af;
    font-size: 0.85rem;
    font-weight: 500;
    cursor: pointer;
    padding: 4px 8px;
    border-radius: 4px;
}
.text-btn:hover {
    color: #ef4444;
    background-color: #fee2e2;
}
.card {
    background: #ffffff;
    border-radius: 8px;
    padding: 24px;
    box-shadow: 0 1px 3px rgba(0,0,0,0.1);
    margin-bottom: 20px;
}
.topic-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 14px;
    border-bottom: 1px solid #f3f4f6;
    padding-bottom: 12px;
    flex-wrap: wrap;
    gap: 8px;
}
.selector-group {
    display: flex;
    align-items: center;
    gap: 8px;
}
.selector-label {
    font-size: 0.85rem;
    font-weight: 700;
    text-transform: uppercase;
    color: #4b5563;
}
select {
    font-size: 0.9rem;
    padding: 6px 10px;
    border-radius: 6px;
    border: 1px solid #d1d5db;
    background-color: #fff;
    color: #1f2937;
    font-weight: 500;
    outline: none;
}
select:focus {
    border-color: #2563eb;
}
.session-stats {
    font-size: 0.8rem;
    color: #6b7280;
    font-weight: 500;
}
.prompt {
    font-size: 1.15rem;
    line-height: 1.6;
    margin-bottom: 24px;
}
.input-row {
    display: flex;
    gap: 12px;
    align-items: center;
}
input[type="number"] {
    font-size: 1.05rem;
    padding: 8px 12px;
    border: 1px solid #d1d5db;
    border-radius: 6px;
    width: 150px;
}
button {
    background-color: #2563eb;
    color: #ffffff;
    border: none;
    padding: 9px 18px;
    border-radius: 6px;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
}
button:hover {
    background-color: #1d4ed8;
}
.banner {
    margin-top: 18px;
    padding: 12px 16px;
    border-radius: 6px;
    font-weight: 600;
}
.banner.correct {
    background-color: #dcfce7;
    color: #15803d;
}
.banner.incorrect {
    background-color: #fee2e2;
    color: #b91c1c;
}
.solution {
    border-left: 4px solid #ef4444;
}
.solution h3 {
    margin-top: 0;
    color: #991b1b;
}
.graph-container {
    background-color: #fafafa;
    border: 1px solid #e5e7eb;
    border-radius: 6px;
    padding: 12px;
    margin: 16px 0 20px 0;
    display: flex;
    justify-content: center;
}
#normal-curve-svg {
    width: 100%;
    max-width: 580px;
    height: auto;
}