* {
    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: #e0e5ec;
    padding: 20px;
}

.back-button {
    text-decoration: none;
    padding: 12px 20px;
    border: none;
    background: #e0e5ec;
    color: #4a5568;
    font-weight: 600;
    border-radius: 10px;
    box-shadow: 6px 6px 12px #b8bec5,
               -6px -6px 12px #ffffff;
    margin-bottom: 20px;
    font-size: 1.1rem;
    transition: all 0.2s ease;
}

.back-button:hover {
    box-shadow: 4px 4px 8px #b8bec5,
               -4px -4px 8px #ffffff;
    color: #2d3748;
}

.back-button:active {
    box-shadow: inset 4px 4px 8px #b8bec5,
                inset -4px -4px 8px #ffffff;
}

.calculator {
    padding: 25px;
    border-radius: 20px;
    background: #e0e5ec;
    box-shadow: 8px 8px 16px #b8bec5,
               -8px -8px 16px #ffffff;
}

.display {
    padding: 20px;
    margin-bottom: 25px;
    border-radius: 15px;
    background: #e0e5ec;
    box-shadow: inset 4px 4px 8px #b8bec5,
                inset -4px -4px 8px #ffffff;
    text-align: right;
    min-height: 100px;
}

.previous-operand {
    font-size: 1.1rem;
    color: #718096;
    min-height: 24px;
}

.current-operand {
    font-size: 2.2rem;
    color: #2d3748;
    margin-top: 10px;
    font-weight: 600;
}

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

button {
    padding: 20px;
    font-size: 1.3rem;
    border: none;
    border-radius: 12px;
    background: #e0e5ec;
    color: #4a5568;
    cursor: pointer;
    font-weight: 600;
    box-shadow: 6px 6px 12px #b8bec5,
               -6px -6px 12px #ffffff;
    transition: all 0.2s ease;
}

button:hover {
    box-shadow: 4px 4px 8px #b8bec5,
               -4px -4px 8px #ffffff;
    color: #2d3748;
}

button:active {
    box-shadow: inset 4px 4px 8px #b8bec5,
                inset -4px -4px 8px #ffffff;
}

.operator {
    color: #4299e1;
}

.equals {
    grid-column: span 2;
    color: #48bb78;
}

.equals:active {
    color: #38a169;
}