:root {
            --bg-color: #f3f4f6;
            --card-bg-color: white;
            --text-color: #1f2937;
            --header-bg-color: #2563eb;
            --header-text-color: white;
            --sub-header-bg-color: #dbeafe;
            --formula-bg-color: #dbeafe;
            --formula-border-color: #3b82f6;
            --border-color: #e5e7eb;
            --accent-color: #3b82f6;
        }
        html.dark {
            --bg-color: #1f2937;
            --card-bg-color: #374151;
            --text-color: #d1d5db;
            --header-bg-color: #60a5fa;
            --header-text-color: #1f2937;
            --sub-header-bg-color: #4b5563;
            --formula-bg-color: #4b5563;
            --formula-border-color: #60a5fa;
            --border-color: #4b5563;
            --accent-color: #93c5fd;
        }
        body {
            font-family: 'Inter', sans-serif;
            background-color: var(--bg-color);
            color: var(--text-color);
            transition: background-color 0.3s, color 0.3s;
        }
        .card {
            background-color: var(--card-bg-color);
            border-radius: 0.75rem;
            box-shadow: 0 4px 6px -1px rgb(0 0 0 / 0.1), 0 2px 4px -2px rgb(0 0 0 / 0.1);
            margin-bottom: 1.5rem;
            overflow: hidden;
            transition: background-color 0.3s;
        }
        .card-header {
            padding: 1rem 1.5rem;
            background-color: var(--header-bg-color);
            color: var(--header-text-color);
            font-size: 1.5rem;
            font-weight: 600;
            transition: background-color 0.3s, color 0.3s;
        }
        .card-content { padding: 1.5rem; }
        details > summary {
            padding: 1rem;
            background-color: var(--sub-header-bg-color);
            cursor: pointer;
            font-weight: 500;
            border-radius: 0.5rem;
            margin-bottom: 0.5rem;
            list-style: none;
            position: relative;
            transition: background-color 0.3s;
        }
        details > summary::-webkit-details-marker { display: none; }
        details > summary::before {
            content: '+';
            position: absolute;
            right: 1rem;
            font-size: 1.5rem;
            font-weight: 600;
            transition: transform 0.2s;
        }
        details[open] > summary::before { transform: rotate(45deg); }
        .theory-content {
            padding: 1rem;
            border: 1px solid var(--border-color);
            border-radius: 0.5rem;
            transition: border-color 0.3s;
        }
        .formula-box {
            background-color: var(--formula-bg-color);
            border-left: 4px solid var(--formula-border-color);
            padding: 1rem;
            margin: 1rem 0;
            border-radius: 0.25rem;
            transition: background-color 0.3s, border-color 0.3s;
        }