:root {
    --primary: #4F46E5;
    --bg: #0F172A;
    --text: #F8FAFC;
    --panel: rgba(30, 41, 59, 0.85);
}

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

body {
    background-color: var(--bg);
    color: var(--text);
    overflow: hidden;
    width: 100vw;
    height: 100vh;
}

#canvas-container {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: 1;
}

#ui-layer {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: 10;
    pointer-events: none; /* Let clicks pass through to canvas unless hitting UI */
    display: flex;
    justify-content: center;
    align-items: center;
}

/* Start Screen */
#start-screen {
    background: var(--panel);
    padding: 40px;
    border-radius: 16px;
    text-align: center;
    backdrop-filter: blur(10px);
    border: 1px solid rgba(255, 255, 255, 0.1);
    pointer-events: auto;
    transition: opacity 0.5s ease;
}

#start-screen h1 {
    margin-bottom: 10px;
    font-size: 2rem;
}

#start-screen p {
    color: #94A3B8;
    margin-bottom: 30px;
}

button {
    background: var(--primary);
    color: white;
    border: none;
    padding: 12px 24px;
    font-size: 1rem;
    border-radius: 8px;
    cursor: pointer;
    transition: transform 0.2s, background 0.2s;
    pointer-events: auto;
}

button:hover {
    background: #4338CA;
    transform: translateY(-2px);
}

/* Control Panel */
#control-panel {
    position: absolute;
    bottom: 40px;
    background: var(--panel);
    padding: 20px 30px;
    border-radius: 16px;
    backdrop-filter: blur(10px);
    border: 1px solid rgba(255, 255, 255, 0.1);
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 15px;
    pointer-events: auto;
    transition: opacity 0.5s ease, transform 0.5s ease;
    transform: translateY(0);
}

#control-panel.hidden {
    opacity: 0;
    transform: translateY(20px);
    pointer-events: none;
}

#instruction-text {
    font-size: 0.9rem;
    color: #94A3B8;
    text-transform: uppercase;
    letter-spacing: 1px;
}

#equation-display {
    font-size: 1.5rem;
    font-weight: bold;
    font-family: 'Courier New', Courier, monospace;
    color: #38BDF8;
}

.slider-container {
    display: flex;
    align-items: center;
    gap: 15px;
    width: 100%;
    margin-top: 10px;
    transition: opacity 0.3s;
}

.slider-container.hidden {
    display: none;
}

input[type=range] {
    flex-grow: 1;
    accent-color: var(--primary);
    cursor: pointer;
}

#slider-value {
    font-family: monospace;
    width: 40px;
    text-align: right;
    font-size: 1.1rem;
}

.button-group {
    display: flex;
    gap: 10px;
    margin-top: 10px;
}

.button-group button.hidden {
    display: none;
}

.hidden {
    display: none !important;
}