/* Basic Reset */
*, *::before, *::after {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    font-family: sans-serif;
}

:root {
    --calc-bg: #333;
    --btn-bg: #555;
    --btn-op-bg: #f8961e;
    --output-bg: #222;
    --text-color: #fff;
    --text-color-light: #aaa;
    --font-size-lg: 2.5rem;
    --font-size-md: 1.5rem;
}

body {
    background-color: #f0f0f0;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
}

/* Header & Footer */
.header {
    background-color: var(--calc-bg);
    color: var(--text-color);
    padding: 1rem 1.5rem;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.header h1 {
    font-size: 1.5rem;
}

.built-by a {
    color: #4dc2ff;
    text-decoration: none;
    font-size: 0.9rem;
    transition: color 0.2s;
}

.built-by a:hover {
    color: #87e0ff;
}

.footer {
    margin-top: auto;
    background-color: var(--calc-bg);
    color: var(--text-color-light);
    text-align: center;
    padding: 0.5rem;
    font-size: 0.8rem;
}

/* Calculator Layout */
.calculator-container {
    flex-grow: 1;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 20px;
}

.calculator-grid {
    display: grid;
    /* Mobile Layout: 4 columns */
    grid-template-columns: repeat(4, 1fr);
    grid-gap: 1px; /* Visual separator between buttons */
    width: 100%;
    max-width: 400px;
    border: 1px solid var(--calc-bg);
    border-radius: 12px;
    overflow: hidden; /* Ensures buttons don't leak out of border radius */
    box-shadow: 0 10px 20px rgba(0, 0, 0, 0.5);
}

/* Output Display */
.output {
    grid-column: 1 / -1;
    background-color: var(--output-bg);
    display: flex;
    flex-direction: column;
    align-items: flex-end;
    justify-content: space-around;
    padding: 10px;
    word-wrap: break-word;
    word-break: break-all;
    min-height: 90px;
}

.output .previous-operand {
    color: var(--text-color-light);
    font-size: var(--font-size-md);
    min-height: 1.5rem;
}

.output .current-operand {
    color: var(--text-color);
    font-size: var(--font-size-lg);
    line-height: 1;
}


/* Buttons */
.calculator-grid button {
    cursor: pointer;
    font-size: 1.5rem;
    border: none;
    outline: none;
    background-color: var(--btn-bg);
    color: var(--text-color);
    padding: 20px 10px;
    transition: background-color 0.1s;
}

.calculator-grid button:hover {
    background-color: #666;
}

/* Specific button styles */
button[data-operation], 
button[data-equals] {
    background-color: var(--btn-op-bg);
    color: var(--text-color);
}

button[data-operation]:hover,
button[data-equals]:hover {
    background-color: #ffb74d;
}

button[data-all-clear] {
    background-color: #d9534f;
}
button[data-all-clear]:hover {
    background-color: #e26965;
}

.span-two {
    grid-column: span 2;
}

/* Desktop / Tablet adjustments */
@media (min-width: 600px) {
    .calculator-grid {
        max-width: 350px;
    }
    
    .header h1 {
        font-size: 2rem;
    }
}