:root {
    --bg-color: #f0f4f8;
    --panel-bg: #ffffff;
    --text-color: #333;
    --accent-color: #3b82f6;
    --tissue-color: #ffe4e1;
    --lung-color: #e0f7fa;
    --vessel-color: #ffcdd2;
    --o2-color: #ef4444; /* Red for Oxygen */
    --co2-color: #3b82f6; /* Blue for CO2 representation (often used in diagrams) or Grey */
    --rbc-color: #b91c1c;
}

body {
    font-family: 'Noto Sans TC', sans-serif;
    background-color: var(--bg-color);
    color: var(--text-color);
    margin: 0;
    padding: 0;
    display: flex;
    flex-direction: column;
    align-items: center;
    min-height: 100vh;
}

.container {
    width: 95%;
    max-width: 1200px;
    margin: 20px auto;
    background: white;
    padding: 20px;
    border-radius: 12px;
    box-shadow: 0 4px 6px rgba(0,0,0,0.1);
}

header {
    text-align: center;
    margin-bottom: 20px;
}

h1 {
    margin: 0;
    color: #1e293b;
}

.main-view {
    display: flex;
    gap: 20px;
    justify-content: space-between;
    margin-bottom: 20px;
}

.panel {
    flex: 1;
    background: var(--panel-bg);
    border: 1px solid #e2e8f0;
    border-radius: 8px;
    padding: 15px;
    display: flex;
    flex-direction: column;
    align-items: center;
}

.tissue-panel {
    background-color: #fff5f5;
}

.lung-panel {
    background-color: #f0f9ff;
}

.svg-container {
    width: 100%;
    height: 400px;
    background: white;
    border-radius: 8px;
    border: 1px solid #cbd5e1;
    position: relative;
    overflow: hidden;
}

svg {
    width: 100%;
    height: 100%;
    display: block;
}

.controls {
    display: flex;
    justify-content: center;
    align-items: center;
    gap: 20px;
    padding: 15px;
    background: #f8fafc;
    border-radius: 8px;
}

button {
    padding: 8px 16px;
    background-color: var(--accent-color);
    color: white;
    border: none;
    border-radius: 4px;
    cursor: pointer;
    font-size: 1rem;
    transition: background-color 0.2s;
}

button:hover {
    background-color: #2563eb;
}

.legend {
    display: flex;
    gap: 15px;
    margin-left: 20px;
}

.item {
    display: flex;
    align-items: center;
    gap: 5px;
    font-size: 0.9rem;
}

.dot {
    width: 12px;
    height: 12px;
    border-radius: 50%;
    display: inline-block;
}

.dot.o2 { background-color: var(--o2-color); }
.dot.co2 { background-color: var(--co2-color); }
.dot.rbc { background-color: var(--rbc-color); }

/* SVG Elements Styling */
.vessel-wall {
    fill: none;
    stroke: #e57373;
    stroke-width: 3;
}

.tissue-cell {
    fill: #ffccbc;
    stroke: #ffab91;
    stroke-width: 2;
}

.cell-nucleus {
    fill: #d84315;
    opacity: 0.6;
}

.alveolus {
    fill: #b3e5fc;
    stroke: #81d4fa;
    stroke-width: 2;
}

.rbc {
    fill: var(--rbc-color);
    transition: fill 0.5s;
}

.particle {
    transition: cx 0.1s linear, cy 0.1s linear;
}

/* Ensure colors override any default */
circle.dot.o2 { fill: var(--o2-color); }
circle.dot.co2 { fill: var(--co2-color); }

.particle-label {
    font-size: 10px;
    fill: #333;
    pointer-events: none;
    text-anchor: middle;
    display: none; /* Hidden by default */
}