:root {
    --bg-color: #140606;
    --calc-bg: rgba(42, 16, 16, 0.6);
    --border-glow: rgba(255, 64, 64, 0.15);
    
    --display-bg: rgba(0, 0, 0, 0.4);
    --display-text-main: #ff4040;
    --display-text-sub: rgba(255, 64, 64, 0.6);
    
    --btn-bg: rgba(60, 22, 22, 0.8);
    --btn-hover: rgba(255, 64, 64, 0.2);
    --btn-active: rgba(255, 64, 64, 0.4);
    --btn-text: #ffffff;
    
    --fn-bg: rgba(168, 0, 0, 0.2);
    --fn-hover: rgba(168, 0, 0, 0.4);
    --fn-text: #ff4040;
    
    --op-bg: rgba(255, 64, 64, 0.8);
    --op-hover: #ff4040;
    --op-text: #000000;
}

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

body {
    background-color: var(--bg-color);
    background-image: 
        radial-gradient(circle at 15% 50%, rgba(255, 64, 64, 0.05), transparent 25%),
        radial-gradient(circle at 85% 30%, rgba(168, 0, 0, 0.08), transparent 25%);
    font-family: 'Inter', sans-serif;
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
}

.calculator-container {
    padding: 2rem;
    position: relative;
    /* Subtle background glow effect behind the calculator */
    box-shadow: 0 0 100px -20px var(--border-glow);
    border-radius: 36px;
}

.calculator {
    width: 320px;
    background: var(--calc-bg);
    backdrop-filter: blur(20px);
    -webkit-backdrop-filter: blur(20px);
    border: 1px solid var(--border-glow);
    border-radius: 28px;
    padding: 1.5rem;
    box-shadow: 
        0 25px 50px -12px rgba(0, 0, 0, 0.5),
        inset 0 1px 1px rgba(255, 255, 255, 0.05);
}

.display {
    background: var(--display-bg);
    border-radius: 16px;
    padding: 1.5rem 1rem;
    margin-bottom: 1.5rem;
    text-align: right;
    display: flex;
    flex-direction: column;
    justify-content: center;
    box-shadow: inset 0 2px 10px rgba(0,0,0,0.5);
    border: 1px solid rgba(255, 64, 64, 0.05);
    word-wrap: break-word;
    word-break: break-all;
    min-height: 110px;
}

.previous-operand {
    font-size: 1rem;
    color: var(--display-text-sub);
    min-height: 1.5rem;
    font-family: 'Orbitron', monospace;
    letter-spacing: 1px;
}

.current-operand {
    font-size: 2.5rem;
    color: var(--display-text-main);
    font-family: 'Orbitron', monospace;
    font-weight: 700;
    text-shadow: 0 0 10px rgba(255, 64, 64, 0.3);
}

.keypad {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    grid-gap: 12px;
}

.btn {
    border: none;
    outline: none;
    font-family: 'Inter', sans-serif;
    font-size: 1.25rem;
    font-weight: 600;
    border-radius: 14px;
    height: 60px;
    cursor: pointer;
    background: var(--btn-bg);
    color: var(--btn-text);
    transition: all 0.2s cubic-bezier(0.175, 0.885, 0.32, 1.275);
    box-shadow: 0 4px 10px rgba(0, 0, 0, 0.2);
    border: 1px solid rgba(255, 255, 255, 0.02);
}

/* Base button hover/active States */
.btn:hover {
    background: var(--btn-hover);
    transform: translateY(-2px);
    box-shadow: 0 6px 15px rgba(255, 64, 64, 0.15);
}

.btn:active {
    background: var(--btn-active);
    transform: translateY(1px) scale(0.95);
}

/* Function buttons (AC, DEL) */
.fn-btn {
    background: var(--fn-bg);
    color: var(--fn-text);
}
.fn-btn:hover {
    background: var(--fn-hover);
    box-shadow: 0 6px 15px rgba(168, 0, 0, 0.2);
}

/* Clear button takes 2 slots */
.btn[data-action="clear"] {
    grid-column: span 2;
}

/* Operator buttons (/, *, -, +) */
.op-btn {
    background: var(--op-bg);
    color: var(--op-text);
    font-size: 1.5rem;
}
.op-btn:hover {
    background: var(--op-hover);
    box-shadow: 0 6px 15px rgba(255, 64, 64, 0.4);
}

/* Zero button spans 2 columns */
.span-2 {
    grid-column: span 2;
}

/* Equals Button Wrapper */
.eq-btn {
    background: var(--op-hover);
    color: var(--bg-color);
    box-shadow: 0 0 15px rgba(255, 64, 64, 0.4);
}
.eq-btn:hover {
    background: #ff6666;
    box-shadow: 0 0 25px rgba(255, 64, 64, 0.6);
}