:root {
    /* Color Palette - Modern Gradient Theme */
    --primary-gradient: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    --secondary-gradient: linear-gradient(135deg, #f093fb 0%, #f5576c 100%);
    --success-gradient: linear-gradient(135deg, #4facfe 0%, #00f2fe 100%);

    /* Dark Theme Colors */
    --bg-primary: #0f0f1e;
    --bg-secondary: #1a1a2e;
    --bg-calculator: #16213e;
    --bg-display: #0f1419;

    /* Text Colors */
    --text-primary: #ffffff;
    --text-secondary: #a0aec0;
    --text-display: #00f2fe;

    /* Button Colors */
    --btn-number-bg: rgba(255, 255, 255, 0.08);
    --btn-number-hover: rgba(255, 255, 255, 0.15);
    --btn-function-bg: rgba(102, 126, 234, 0.2);
    --btn-function-hover: rgba(102, 126, 234, 0.35);
    --btn-operator-bg: rgba(245, 87, 108, 0.2);
    --btn-operator-hover: rgba(245, 87, 108, 0.35);

    /* Shadows */
    --shadow-sm: 0 2px 8px rgba(0, 0, 0, 0.3);
    --shadow-md: 0 4px 16px rgba(0, 0, 0, 0.4);
    --shadow-lg: 0 8px 32px rgba(0, 0, 0, 0.5);
    --shadow-glow: 0 0 20px rgba(102, 126, 234, 0.3);

    /* Spacing */
    --spacing-xs: 0.5rem;
    --spacing-sm: 1rem;
    --spacing-md: 1.5rem;
    --spacing-lg: 2rem;
    --spacing-xl: 3rem;

    /* Border Radius */
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 20px;
    --radius-xl: 24px;

    /* Transitions */
    --transition-fast: 0.15s ease;
    --transition-normal: 0.3s ease;
    --transition-slow: 0.5s ease;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    min-height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: var(--spacing-md);
    overflow: hidden;
    position: relative;
}

/* Animated Background */
body::before {
    content: '';
    position: absolute;
    top: -50%;
    left: -50%;
    width: 200%;
    height: 200%;
    background: radial-gradient(circle at 20% 50%, rgba(102, 126, 234, 0.1) 0%, transparent 50%),
        radial-gradient(circle at 80% 80%, rgba(245, 87, 108, 0.1) 0%, transparent 50%),
        radial-gradient(circle at 40% 20%, rgba(0, 242, 254, 0.05) 0%, transparent 50%);
    animation: backgroundMove 20s ease-in-out infinite;
    pointer-events: none;
}

@keyframes backgroundMove {

    0%,
    100% {
        transform: translate(0, 0) rotate(0deg);
    }

    33% {
        transform: translate(5%, 5%) rotate(120deg);
    }

    66% {
        transform: translate(-5%, 5%) rotate(240deg);
    }
}

.container {
    position: relative;
    z-index: 1;
    width: 100%;
    max-width: 420px;
}

.calculator {
    background: var(--bg-calculator);
    border-radius: var(--radius-xl);
    padding: var(--spacing-lg);
    box-shadow: var(--shadow-lg), var(--shadow-glow);
    backdrop-filter: blur(10px);
    border: 1px solid rgba(255, 255, 255, 0.1);
    transition: transform var(--transition-normal), box-shadow var(--transition-normal);
    animation: fadeIn 0.6s ease;
    will-change: transform, box-shadow;
}

.calculator:hover {
    transform: translateY(-4px);
    box-shadow: 0 12px 48px rgba(0, 0, 0, 0.6), 0 0 30px rgba(102, 126, 234, 0.4);
}


.calculator-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: var(--spacing-md);
}

.calculator-header h1 {
    font-size: 1.5rem;
    font-weight: 700;
    background: var(--primary-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.theme-indicator {
    width: 12px;
    height: 12px;
    border-radius: 50%;
    background: var(--success-gradient);
    box-shadow: 0 0 12px rgba(0, 242, 254, 0.6);
    animation: pulse 2s ease-in-out infinite;
}

@keyframes pulse {

    0%,
    100% {
        opacity: 1;
        transform: scale(1);
    }

    50% {
        opacity: 0.6;
        transform: scale(1.1);
    }
}

.display-container {
    background: var(--bg-display);
    border-radius: var(--radius-lg);
    padding: var(--spacing-md);
    margin-bottom: var(--spacing-lg);
    min-height: 120px;
    display: flex;
    flex-direction: column;
    justify-content: flex-end;
    border: 1px solid rgba(255, 255, 255, 0.05);
    position: relative;
    overflow: hidden;
}

.display-container::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    height: 1px;
    background: linear-gradient(90deg, transparent, rgba(0, 242, 254, 0.5), transparent);
}

.display-history {
    font-size: 0.875rem;
    color: var(--text-secondary);
    min-height: 1.5rem;
    margin-bottom: var(--spacing-xs);
    text-align: right;
    opacity: 0.7;
    font-weight: 500;
}

.display-main {
    font-size: 3rem;
    font-weight: 700;
    text-align: right;
    color: var(--text-display);
    text-shadow: 0 0 20px rgba(0, 242, 254, 0.3);
    letter-spacing: -0.02em;
    word-break: break-all;
    line-height: 1.2;
    transition: all var(--transition-fast);
}

.display-main.error {
    color: #f5576c;
    text-shadow: 0 0 20px rgba(245, 87, 108, 0.5);
    animation: shake 0.5s ease;
}

@keyframes shake {

    0%,
    100% {
        transform: translateX(0);
    }

    25% {
        transform: translateX(-10px);
    }

    75% {
        transform: translateX(10px);
    }
}

.button-grid {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: var(--spacing-sm);
    margin-bottom: var(--spacing-md);
}

.btn {
    background: var(--btn-number-bg);
    border: 1px solid rgba(255, 255, 255, 0.1);
    border-radius: var(--radius-md);
    color: var(--text-primary);
    font-size: 1.5rem;
    font-weight: 600;
    font-family: 'Inter', sans-serif;
    padding: var(--spacing-md);
    cursor: pointer;
    transition: all var(--transition-fast);
    position: relative;
    overflow: hidden;
    min-height: 64px;
    display: flex;
    align-items: center;
    justify-content: center;
}

.btn::before {
    content: '';
    position: absolute;
    top: 50%;
    left: 50%;
    width: 0;
    height: 0;
    border-radius: 50%;
    background: rgba(255, 255, 255, 0.2);
    transform: translate(-50%, -50%);
    transition: width var(--transition-normal), height var(--transition-normal);
}

.btn:hover::before {
    width: 300px;
    height: 300px;
}

.btn:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
    border-color: rgba(255, 255, 255, 0.2);
}

.btn:active {
    transform: translateY(0) scale(0.95);
}

.btn-number {
    background: var(--btn-number-bg);
}

.btn-number:hover {
    background: var(--btn-number-hover);
}

.btn-function {
    background: var(--btn-function-bg);
    color: #667eea;
    font-weight: 700;
}

.btn-function:hover {
    background: var(--btn-function-hover);
    color: #ffffff;
}

.btn-operator {
    background: var(--btn-operator-bg);
    color: #f5576c;
    font-weight: 700;
}

.btn-operator:hover {
    background: var(--btn-operator-hover);
    color: #ffffff;
}

.btn-operator.active {
    background: var(--secondary-gradient);
    color: white;
    box-shadow: 0 0 20px rgba(245, 87, 108, 0.5);
}

.btn-equals {
    background: var(--success-gradient);
    color: white;
    font-weight: 700;
    box-shadow: 0 4px 16px rgba(0, 242, 254, 0.3);
}

.btn-equals:hover {
    box-shadow: 0 6px 24px rgba(0, 242, 254, 0.5);
    transform: translateY(-3px);
}

.btn-zero {
    grid-column: span 2;
}

.calculator-footer {
    text-align: center;
    color: var(--text-secondary);
    font-size: 0.75rem;
    font-weight: 500;
    opacity: 0.6;
    padding-top: var(--spacing-sm);
    border-top: 1px solid rgba(255, 255, 255, 0.05);
}

/* Responsive Design */
@media (max-width: 480px) {
    .calculator {
        padding: var(--spacing-md);
    }

    .display-main {
        font-size: 2.5rem;
    }

    .btn {
        font-size: 1.25rem;
        min-height: 56px;
        padding: var(--spacing-sm);
    }

    .button-grid {
        gap: var(--spacing-xs);
    }
}

@media (max-width: 360px) {
    .display-main {
        font-size: 2rem;
    }

    .btn {
        font-size: 1.125rem;
        min-height: 48px;
    }
}

/* Accessibility */
.btn:focus {
    outline: 2px solid rgba(102, 126, 234, 0.6);
    outline-offset: 2px;
}

.btn:focus:not(:focus-visible) {
    outline: none;
}

/* Loading Animation */
@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(20px);
    }

    to {
        opacity: 1;
        transform: translateY(0);
    }
}