:root {
            --bg-color: #04070a;
            --container-bg: #090f16;
            --card-bg: #0e161f;
            --border-color: #1c2b38;
            --accent-blue: #3b82f6;
            --accent-cyan: #22d3ee;
            --accent-green: #4ade80;
            --accent-amber: #fbbf24;
            --accent-red: #f87171;
            --text-primary: #e3edf5;
            --text-secondary: #6f8598;
            --font-display: 'Audiowide', sans-serif;
            --font-body: 'Titillium Web', sans-serif;
            --font-mono: 'Fira Code', monospace;
            --glow-color: rgba(59, 130, 246, 0.25);
            --shadow-color: rgba(0, 0, 0, 0.65);
            --panel-sunken: rgba(0, 0, 0, 0.32);

            /* Shared kit mapping — dark navy ink reads better on the
               brighter dark-mode blue accent than white does */
            --kit-accent: var(--accent-blue);
            --kit-accent-ink: #051224;
            --kit-accent-glow: var(--glow-color);
            --kit-surface: var(--card-bg);
            --kit-border: var(--border-color);
            --kit-input-bg: var(--container-bg);
            --kit-error: var(--accent-red);
        }

        /* TRUE LIGHT THEME — a bright, clean daytime test-track control
           room look, not a second dark palette. Backgrounds sit in the
           #f2f5f8–#ffffff range with dark, readable text throughout. */
        body.light-mode {
            --bg-color: #f1f4f8;
            --container-bg: #ffffff;
            --card-bg: #f6f9fc;
            --border-color: #ccd8e3;
            --accent-blue: #1d4ed8;
            --accent-cyan: #0e7490;
            --accent-green: #15803d;
            --accent-amber: #b45309;
            --accent-red: #dc2626;
            --text-primary: #0c1a26;
            --text-secondary: #4d6377;
            --glow-color: rgba(29, 78, 216, 0.14);
            --shadow-color: rgba(10, 30, 50, 0.1);
            --panel-sunken: rgba(10, 30, 50, 0.05);

            /* Shared kit mapping (light) — redeclared in full: a custom
               property whose value is var(--other-var) does not reliably
               re-track that other var's light-mode override, so every
               --kit-* token must be restated here, not just the ink. White
               ink reads well on the deeper light-mode blue accent. */
            --kit-accent: var(--accent-blue);
            --kit-accent-ink: #ffffff;
            --kit-accent-glow: rgba(29, 78, 216, 0.16);
            --kit-surface: var(--card-bg);
            --kit-border: var(--border-color);
            --kit-input-bg: var(--container-bg);
            --kit-error: var(--accent-red);
        }

        * {
            box-sizing: border-box;
            margin: 0;
            padding: 0;
            transition: background-color 0.25s ease, color 0.25s ease, border-color 0.25s ease;
        }

        body {
            background-color: var(--bg-color);
            color: var(--text-primary);
            font-family: var(--font-body);
            min-height: 100vh;
            display: flex;
            flex-direction: column;
            align-items: center;
            justify-content: center;
            padding: 20px;
        }

        .top-controls {
            position: fixed;
            top: 20px;
            right: 24px;
            z-index: 1000;
        }

        .theme-toggle-btn {
            background: var(--container-bg);
            color: var(--text-primary);
            border: 2px solid var(--border-color);
            padding: 8px 16px;
            border-radius: 4px;
            font-family: var(--font-mono);
            font-weight: 600;
            font-size: 0.85rem;
            cursor: pointer;
            box-shadow: 0 4px 10px var(--shadow-color);
            display: flex;
            align-items: center;
            gap: 6px;
        }

        .theme-toggle-btn:hover {
            border-color: var(--accent-blue);
            color: var(--accent-blue);
        }

        .hud-container {
            width: 100%;
            max-width: 860px;
            background: var(--container-bg);
            border: 2px solid var(--border-color);
            border-top: 6px solid var(--accent-blue);
            border-radius: 8px;
            padding: 36px;
            box-shadow: 0 16px 40px var(--shadow-color);
            min-height: 600px;
            display: flex;
            flex-direction: column;
            justify-content: space-between;
        }

        .hud-title {
            font-family: var(--font-display);
            font-size: 1.55rem;
            color: var(--accent-blue);
            text-align: center;
            letter-spacing: 0.5px;
            margin-bottom: 8px;
        }

        .hud-subtitle {
            font-family: var(--font-mono);
            text-align: center;
            color: var(--text-secondary);
            font-size: 0.88rem;
            margin-bottom: 22px;
            letter-spacing: 0.5px;
            text-transform: uppercase;
        }

        .status-bar {
            display: flex;
            justify-content: space-between;
            align-items: center;
            background: var(--panel-sunken);
            border: 1px solid var(--border-color);
            padding: 12px 20px;
            margin-bottom: 20px;
            border-radius: 6px;
            font-family: var(--font-mono);
            flex-wrap: wrap;
            gap: 8px;
        }

        .status-item {
            display: flex;
            align-items: center;
            gap: 8px;
            font-size: 0.9rem;
            font-weight: 600;
        }

        .status-label {
            color: var(--text-secondary);
        }

        .status-value {
            color: var(--accent-blue);
            font-size: 1rem;
        }

        /* Discriminant Status Light + Equation Calc Pad */
        .calc-pad {
            background: var(--card-bg);
            border: 1px solid var(--border-color);
            border-radius: 8px;
            padding: 18px 22px;
            margin-bottom: 16px;
        }

        .status-light-row {
            display: flex;
            align-items: center;
            gap: 10px;
            margin-bottom: 14px;
        }

        .status-light {
            width: 16px;
            height: 16px;
            border-radius: 50%;
            box-shadow: 0 0 10px currentColor;
        }

        .light-neutral { background: #64748b; color: #64748b; }
        .light-green { background: var(--accent-green); color: var(--accent-green); }
        .light-amber { background: var(--accent-amber); color: var(--accent-amber); }
        .light-red { background: var(--accent-red); color: var(--accent-red); }

        .status-light-label {
            font-family: var(--font-mono);
            font-size: 0.78rem;
            font-weight: 600;
            letter-spacing: 1px;
            color: var(--text-secondary);
            text-transform: uppercase;
        }

        .equation-card {
            border-left: 3px solid var(--accent-blue);
            padding-left: 16px;
        }

        .equation-tag {
            font-family: var(--font-mono);
            font-size: 0.7rem;
            text-transform: uppercase;
            color: var(--accent-blue);
            font-weight: 600;
            margin-bottom: 8px;
            letter-spacing: 1px;
        }

        .equation-text {
            font-size: 1.3rem;
            font-weight: 600;
            line-height: 1.5;
        }

        .prompt-line {
            font-family: var(--font-body);
            font-size: 1.02rem;
            font-weight: 600;
            margin-bottom: 14px;
            color: var(--text-primary);
        }

        /* PHASE 1 — Type-the-calculation input row */
        .calc-input-row {
            display: flex;
            gap: 12px;
            flex-wrap: wrap;
            align-items: stretch;
            margin-bottom: 8px;
        }

        .calc-input {
            flex: 1 1 200px;
            min-width: 160px;
            font-family: var(--font-mono);
            font-size: 1.1rem;
            font-weight: 600;
            padding: 14px;
            border-radius: 4px;
            border: 1px solid var(--border-color);
            background: var(--card-bg);
            color: var(--text-primary);
        }

        .calc-input:focus {
            outline: none;
            border-color: var(--accent-blue);
            box-shadow: 0 0 0 3px var(--glow-color);
        }

        .calc-input:disabled {
            opacity: 0.75;
        }

        .calc-submit-btn {
            margin-top: 0;
            flex: 0 0 auto;
            white-space: nowrap;
        }

        .calc-submit-btn:disabled {
            opacity: 0.6;
            cursor: default;
        }

        .calc-input-error {
            display: none;
            color: var(--accent-red);
            font-family: var(--font-mono);
            font-size: 0.85rem;
            margin-bottom: 8px;
        }

        .calc-feedback {
            font-family: var(--font-mono);
            font-size: 0.92rem;
            font-weight: 600;
            min-height: 22px;
            margin-bottom: 6px;
        }

        .calc-feedback.text-success { color: var(--accent-green); }
        .calc-feedback.text-error { color: var(--accent-red); }

        /* PHASE 2 — Type-Then-Brake QTE gauge */
        .qte-phase {
            margin-top: 4px;
        }

        .qte-hint {
            font-family: var(--font-mono);
            text-align: center;
            font-weight: 700;
            font-size: 0.92rem;
            letter-spacing: 0.4px;
            text-transform: uppercase;
            margin-bottom: 10px;
        }

        .brake-gauge {
            display: block;
            width: 100%;
            max-width: 480px;
            height: auto;
            margin: 0 auto;
        }

        .brake-btn {
            display: block;
            margin: 16px auto 8px;
            background: var(--accent-red);
        }

        .brake-btn:disabled {
            opacity: 0.6;
            cursor: default;
        }

        .qte-tip {
            text-align: center;
            font-family: var(--font-mono);
            font-size: 0.78rem;
            color: var(--text-secondary);
            margin-bottom: 6px;
        }

        /* ROUND RESULT — car vs. cone mini-animation */
        .track-stage {
            margin: 14px 0 6px;
        }

        .track-road {
            position: relative;
            height: 64px;
            background: var(--panel-sunken);
            border: 1px solid var(--border-color);
            border-radius: 8px;
            overflow: hidden;
        }

        .track-road::before {
            content: '';
            position: absolute;
            top: 50%;
            left: 0;
            right: 0;
            height: 2px;
            background: repeating-linear-gradient(90deg, var(--border-color) 0 16px, transparent 16px 32px);
            transform: translateY(-50%);
        }

        .track-car {
            position: absolute;
            left: 2%;
            top: 50%;
            font-size: 2rem;
            line-height: 1;
            transform: translate(0, -50%);
            transition: left 0.9s cubic-bezier(0.2, 0.8, 0.3, 1);
            z-index: 2;
        }

        .track-car.car-crash {
            animation: crash-shake 0.5s ease;
        }

        .track-hazard {
            position: absolute;
            left: 80%;
            top: 50%;
            font-size: 2rem;
            line-height: 1;
            transform: translate(-50%, -50%);
            z-index: 1;
        }

        @keyframes crash-shake {
            0%, 100% { transform: translate(0, -50%) rotate(0deg); }
            20% { transform: translate(-4px, -55%) rotate(-8deg); }
            40% { transform: translate(4px, -46%) rotate(7deg); }
            60% { transform: translate(-3px, -52%) rotate(-4deg); }
            80% { transform: translate(3px, -48%) rotate(4deg); }
        }

        .feedback-line {
            font-family: var(--font-mono);
            text-align: center;
            font-size: 1rem;
            font-weight: 600;
            min-height: 24px;
            text-transform: uppercase;
        }

        .text-success { color: var(--accent-green); }
        .text-error { color: var(--accent-red); }

        .screen-center {
            display: flex;
            flex-direction: column;
            align-items: center;
            justify-content: center;
            text-align: center;
            padding: 10px 0;
        }

        .action-btn {
            background: var(--accent-blue);
            color: #ffffff;
            border: none;
            padding: 14px 36px;
            font-family: var(--font-display);
            font-size: 0.95rem;
            text-transform: uppercase;
            letter-spacing: 1px;
            cursor: pointer;
            margin-top: 20px;
            border-radius: 4px;
            box-shadow: 0 4px 12px var(--shadow-color);
        }

        .action-btn:hover {
            opacity: 0.9;
        }

        .brief-box {
            background: var(--card-bg);
            border: 1px solid var(--border-color);
            padding: 20px 28px;
            text-align: left;
            margin: 20px 0;
            max-width: 680px;
            line-height: 1.7;
            font-size: 0.95rem;
            border-radius: 6px;
        }

        .brief-box ul {
            padding-left: 20px;
        }

        .brief-box li {
            margin-bottom: 8px;
        }

        footer {
            margin-top: 24px;
            font-family: var(--font-mono);
            font-size: 0.8rem;
            color: var(--text-secondary);
            text-align: center;
            letter-spacing: 0.5px;
        }