:root {
    --primary: #4e54c8;
    --primary-dark: #3f44a5;
    --accent: #8f94fb;
    --bg-gradient: linear-gradient(135deg, #f5f7fa 0%, #c3cfe2 100%);
    --surface: #ffffff;
    --text: #333333;
    --text-light: #777777;
    --success: #2ecc71;
    --danger: #e74c3c;
    --shadow: 0 10px 20px rgba(0,0,0,0.1);
    --radius: 16px;
}

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

body {
    background: var(--bg-gradient);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    color: var(--text);
}

.app-container {
    background: var(--surface);
    width: 90%;
    max-width: 400px;
    border-radius: var(--radius);
    box-shadow: var(--shadow);
    overflow: hidden;
    padding: 2rem;
    position: relative;
}

/* Header & Score */
.game-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 2rem;
}

.logo {
    font-size: 1.5rem;
    font-weight: 700;
}
.highlight { color: var(--primary); }

.score-board {
    display: flex;
    gap: 10px;
    font-size: 0.85rem;
}

.score-item {
    display: flex;
    flex-direction: column;
    align-items: center;
    font-weight: bold;
}
.score-item.success { color: var(--success); }
.score-item.danger { color: var(--danger); }
.score-item .label { font-size: 0.6rem; text-transform: uppercase; color: var(--text-light); }

/* Screens (Menu vs Game) */
.screen { display: none; animation: fadeIn 0.3s ease; }
.screen.active { display: block; }
@keyframes fadeIn { from { opacity: 0; transform: translateY(10px); } to { opacity: 1; transform: translateY(0); } }

/* Menu Styles */
h2 { text-align: center; margin-bottom: 1.5rem; color: var(--primary); }

.difficulty-grid {
    display: flex;
    flex-direction: column;
    gap: 10px;
    margin-bottom: 1.5rem;
}

.options-row {
    display: flex;
    justify-content: space-between;
    gap: 10px;
}

/* Buttons */
.btn {
    border: none;
    padding: 12px;
    border-radius: 8px;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.2s;
    font-size: 1rem;
}

.btn-outline {
    background: transparent;
    border: 2px solid var(--primary);
    color: var(--primary);
}
.btn-outline:hover { background: var(--primary); color: white; }

.btn-primary { background: var(--primary); color: white; width: 100%; }
.btn-primary:hover { background: var(--primary-dark); }

.btn-secondary { background: #eee; color: var(--text-light); flex-grow: 1; }
.btn-secondary.active { background: var(--accent); color: white; }

.btn-text { background: transparent; color: var(--text-light); }
.btn-text:hover { color: var(--danger); }

/* Game Area */
.question-container { text-align: center; margin-bottom: 1rem; }
.q-label { font-size: 0.8rem; color: var(--text-light); text-transform: uppercase; letter-spacing: 1px; }
.math-problem { font-size: 2.5rem; font-weight: 700; margin-top: 5px; color: var(--text); }

/* Timer */
.timer-container {
    width: 100%;
    height: 6px;
    background: #eee;
    border-radius: 3px;
    margin-bottom: 1.5rem;
    overflow: hidden;
}
.timer-bar {
    height: 100%;
    background: var(--primary);
    width: 100%;
    transition: width 0.1s linear;
}

/* Inputs */
.input-area {
    display: flex;
    gap: 10px;
    margin-bottom: 1.5rem;
}
#user-input {
    flex-grow: 1;
    font-size: 1.5rem;
    padding: 10px;
    border: 2px solid #eee;
    border-radius: 8px;
    text-align: center;
    color: var(--primary);
    font-weight: bold;
    outline: none;
}
.btn-icon {
    background: #eee;
    border: none;
    width: 50px;
    border-radius: 8px;
    font-size: 1.2rem;
    cursor: pointer;
}
.btn-icon:hover { background: #ddd; }

/* Numpad */
.numpad {
    display: grid;
    grid-template-columns: repeat(4, 1fr); /* 4 columns for +, -, *, etc layout */
    gap: 10px;
    margin-bottom: 1.5rem;
}
.num-btn {
    background: #f8f9fa;
    border: 1px solid #e9ecef;
    padding: 15px 0;
    border-radius: 8px;
    font-size: 1.2rem;
    font-weight: 600;
    cursor: pointer;
    transition: transform 0.1s;
}
.num-btn:active { transform: scale(0.95); background: #e2e6ea; }

.action-row { display: flex; justify-content: space-between; align-items: center; gap: 1rem; }