:root {
    --canvas-bg: #0a0a0a;
    --calc-body: #141414;
    --display-bg: #000000;
    --text-main: #e5e5e5;
    --text-dim: #737373;
    --accent-silver: #ffffff;
    --btn-neutral: #1d1d1d;
    --btn-hover: #262626;
    --operator-bg: #222222;
    --equal-bg: #f5f5f5;
    --equal-text: #000000;
    --border-subtle: 1px solid rgba(255, 255, 255, 0.05);
    --shadow-tactile: 0 10px 30px rgba(0, 0, 0, 0.8), inset 0 1px 1px rgba(255, 255, 255, 0.05);
    --transition-fast: 0.15s cubic-bezier(0.4, 0, 0.2, 1);
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Poppins', sans-serif;
}

body {
    background-color: var(--canvas-bg);
    display: flex;
    justify-content: center;
    align-items: center;
    overflow: hidden;
    height: 100vh;
}

/* Main Container */
.calculator-container {
    width: 400px;
    background: var(--calc-body);
    padding: 32px 24px;
    border-radius: 40px;
    border: var(--border-subtle);
    box-shadow: var(--shadow-tactile);
}

/* Display */
.display {
    background: var(--display-bg);
    margin-bottom: 32px;
    padding: 24px;
    border-radius: 24px;
    text-align: right;
    min-height: 120px;
    display: flex;
    flex-direction: column;
    justify-content: flex-end;
    border: 1px solid rgba(255, 255, 255, 0.03);
    box-shadow: inset 0 4px 12px rgba(0, 0, 0, 0.5);
}

.previous-operand {
    color: var(--text-dim);
    font-size: 0.9rem;
    margin-bottom: 8px;
    font-weight: 300;
}

.current-operand {
    color: var(--text-main);
    font-size: 2.8rem;
    font-weight: 500;
}

/* Grid Layout */
.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 16px;
}

/* Tactile Buttons */
.btn {
    border: var(--border-subtle);
    outline: none;
    background: var(--btn-neutral);
    color: var(--text-main);
    font-size: 1.2rem;
    padding: 10px;
    border-radius: 14px;
    cursor: pointer;
    transition: var(--transition-fast);
    display: flex;
    align-items: center;
    justify-content: center;
    font-weight: 400;
    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.2);
}

/* Hover & Active States */
.btn:hover {
    background: var(--btn-hover);
    transform: translateY(-2px);
    border-color: rgba(255, 255, 255, 0.1);
}

.btn:active {
    transform: translateY(1px);
    background: var(--display-bg);
    box-shadow: inset 0 2px 4px rgba(0, 0, 0, 0.5);
}

/* Special Button Tones */
.btn.operator {
    background: var(--operator-bg);
    color: var(--accent-silver);
    font-size: 1.5rem;
}

.btn.action {
    color: #a3a3a3;
    font-size: 1.1rem;
    font-weight: 300;
}

/* The "Equal" Button */
.btn.equal {
    background: var(--equal-bg);
    color: var(--equal-text);
    border-radius: 24px;
    font-weight: 600;
}

.btn.equal:hover {
    background: #ffffff;
    box-shadow: 0 0 20px rgba(255, 255, 255, 0.15);
}

/* Layout Utilities */
.col-2 {
    grid-column: span 2;
    aspect-ratio: auto;
    height: 70px;
}

/* Smooth icons adjustment */
.fa-backspace {
    font-size: 1rem;
}