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

body {
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    font-family: 'Arial', sans-serif;
}

.calculator {
    background-color: #1e1e1e;
    border-radius: 20px;
    padding: 20px;
    box-shadow: 0 20px 60px rgba(0, 0, 0, 0.3);
    width: 320px;
}

.display {
    margin-bottom: 20px;
}

#display {
    width: 100%;
    padding: 20px;
    font-size: 32px;
    border: none;
    border-radius: 10px;
    background-color: #2d2d2d;
    color: #00ff00;
    text-align: right;
    font-weight: bold;
    cursor: default;
}

#display:focus {
    outline: none;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
}

.btn {
    padding: 20px;
    font-size: 18px;
    font-weight: bold;
    border: none;
    border-radius: 10px;
    background-color: #3d3d3d;
    color: #fff;
    cursor: pointer;
    transition: all 0.2s ease;
}

.btn:hover {
    background-color: #4d4d4d;
    transform: scale(1.05);
}

.btn:active {
    transform: scale(0.95);
}

.btn.operator {
    background-color: #667eea;
}

.btn.operator:hover {
    background-color: #5568d3;
}

.btn.clear {
    background-color: #ff6b6b;
    grid-column: span 1;
}

.btn.clear:hover {
    background-color: #ff5252;
}

.btn.equals {
    background-color: #51cf66;
    grid-column: span 1;
}

.btn.equals:hover {
    background-color: #37b24d;
}

.btn.zero {
    grid-column: span 2;
}

/* Responsive design */
@media (max-width: 600px) {
    .calculator {
        width: 100%;
        max-width: 320px;
        margin: 10px;
    }
}