* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Inter', system-ui, -apple-system, sans-serif;
}

body {
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    background: #e0e0e3;
    padding: 2rem;
}

.back-button {
    text-decoration: none;
    padding: 0.75rem 1.5rem;
    background: #ffffff;
    color: #666;
    border-radius: 15px;
    font-size: 1rem;
    margin-bottom: 2rem;
    border: none;
    box-shadow: 
        8px 8px 16px rgba(163, 177, 198, 0.6),
        -8px -8px 16px rgba(255, 255, 255, 0.8),
        inset -1px -1px 1px rgba(163, 177, 198, 0.2),
        inset 1px 1px 1px rgba(255, 255, 255, 0.8);
    transition: all 0.2s ease;
}

.back-button:hover {
    transform: translateY(-2px);
    box-shadow: 
        10px 10px 20px rgba(163, 177, 198, 0.6),
        -10px -10px 20px rgba(255, 255, 255, 0.8),
        inset -1px -1px 1px rgba(163, 177, 198, 0.2),
        inset 1px 1px 1px rgba(255, 255, 255, 0.8);
}

.calculator {
    background: #f0f0f3;
    padding: 2rem;
    border-radius: 30px;
    box-shadow: 
        20px 20px 40px rgba(143, 157, 178, 0.6),
        -20px -20px 40px rgba(255, 255, 255, 0.8),
        inset -2px -2px 2px rgba(143, 157, 178, 0.2),
        inset 2px 2px 2px rgba(255, 255, 255, 0.8);
}

.display {
    background: #e0e0e3;
    padding: 1.5rem;
    margin-bottom: 1.5rem;
    border-radius: 20px;
    text-align: right;
    min-height: 100px;
    box-shadow: 
        inset 8px 8px 16px rgba(143, 157, 178, 0.4),
        inset -8px -8px 16px rgba(255, 255, 255, 0.6);
}

.previous-operand {
    font-size: 1.1rem;
    color: #777;
    min-height: 1.5rem;
}

.current-operand {
    font-size: 2.2rem;
    color: #444;
    margin-top: 0.5rem;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 15px;
}

button {
    font-size: 1.25rem;
    padding: 1rem;
    border: none;
    background: #f0f0f3;
    color: #444;
    border-radius: 15px;
    cursor: pointer;
    box-shadow: 
        8px 8px 16px rgba(143, 157, 178, 0.6),
        -8px -8px 16px rgba(255, 255, 255, 0.8),
        inset -1px -1px 1px rgba(143, 157, 178, 0.2),
        inset 1px 1px 1px rgba(255, 255, 255, 0.8);
    transition: all 0.2s ease;
}

button:hover {
    transform: translateY(-2px);
    box-shadow: 
        10px 10px 20px rgba(163, 177, 198, 0.6),
        -10px -10px 20px rgba(255, 255, 255, 0.8),
        inset -1px -1px 1px rgba(163, 177, 198, 0.2),
        inset 1px 1px 1px rgba(255, 255, 255, 0.8);
}

button:active {
    transform: translateY(0);
    box-shadow: 
        inset 8px 8px 16px rgba(163, 177, 198, 0.4),
        inset -8px -8px 16px rgba(255, 255, 255, 0.6);
}

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

[data-action="="] {
    background: #e0ebfd;
    color: #2970cc;
}

[data-action="="]:hover {
    background: #d8e5fc;
}