:root {
    --primary: #014a47;
    --primary-light: #eef2ff;
    --bg: #014a47;
    --card: #ffffff;
    --text: #1e293b;
    --border: #e2e8f0;
}

* { box-sizing: border-box; }

body {
    font-family: 'Segoe UI', Tahoma, sans-serif;
    background-color: var(--bg);
    margin: 0;
    padding: 20px;
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    background: var(--card);
    padding: 2rem;
    border-radius: 1.5rem;
    box-shadow: 0 10px 25px rgba(0,0,0,0.05);
}

header { text-align: center; margin-bottom: 2rem; }

.main-layout {
    display: grid;
    grid-template-columns: 320px 1fr;
    gap: 2rem;
}

.controls {
    background: #f1f5f9;
    padding: 1.5rem;
    border-radius: 1rem;
    display: flex;
    flex-direction: column;
    gap: 1rem;
}

.input-group label {
    display: block;
    font-size: 0.85rem;
    font-weight: bold;
    margin-bottom: 0.5rem;
}

select, input {
    width: 100%;
    padding: 0.7rem;
    border: 1px solid var(--border);
    border-radius: 0.5rem;
    font-size: 1rem;
}

.results-card {
    background: var(--primary-light);
    border: 1px solid var(--primary);
    padding: 1rem;
    border-radius: 0.5rem;
    text-align: center;
}

#half-life-value { font-weight: 800; font-size: 1.2rem; }

button {
    background: var(--primary);
    color: white;
    border: none;
    padding: 1rem;
    border-radius: 0.5rem;
    font-weight: bold;
    cursor: pointer;
}

.chart-container {
    height: 400px;
    background: white;
    border: 1px solid var(--border);
    border-radius: 1rem;
    margin-bottom: 1.5rem;
    padding: 10px;
}

.theory-card {
    background: white;
    border-left: 6px solid var(--primary);
    padding: 1.5rem;
    border-radius: 0.5rem;
    box-shadow: 0 4px 6px rgba(0,0,0,0.05);
    line-height: 1.6;
}

.formula-display {
    display: block;
    margin: 1rem 0;
    font-size: 1.2rem;
    color: #c026d3;
}

.example-box {
    margin-top: 1rem;
    padding: 1rem;
    background: #fffbeb;
    border-left: 4px solid #f59e0b;
    font-style: italic;
}

@media (max-width: 900px) { .main-layout { grid-template-columns: 1fr; } }