style.css

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    margin: 0;
    background: #f0f0f3;
    font-family:'Times New Roman', Times, serif;
}

.calculator {
    width: 320px;
    background: #000000;
    border-radius: 15px;
    overflow: hidden;
    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.1);
}

.b
{
    background: #A5A5A5;
    color: #000000;
    font-size: 2rem;
    border: none;
    outline: none;
    padding: 20px;
    cursor: pointer;
    transition: background 0.2s;
    border-radius: 100px;
    border-spacing: 2px;
    font-size: 20px;
}

.display {
    background: #000000;
    color: #fff;
    font-size: 2.5rem;
    text-align: right;
    padding: 20px;
    box-sizing: border-box;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 6px;
    padding: 10px;
}

.btn {
    background: #333333;
    color: #fff;
    border: none;
    outline: none;
    padding: 22px;
    cursor: pointer;
    transition: background 0.2s;
    border-radius: 100px;
    border-spacing: 2px;
    font-size: 25px;
}

.zero
{
    background: #333333;
    color: #fff;
    font-size: 1.7rem;
    border: none;
    outline: none;
    padding: 20px;
    cursor: pointer;
    transition: background 0.2s;
    border-radius: 50px;
    border-spacing: 2px;
}

.b:hover
{
    background: #f0f0f3;
}

.btn:hover,.zero:hover {
    background: #646464;
}

.operator {
    background: #f29925;
}

.operator:hover {
    background: #fbb94e;
}

.zero {
    grid-column: span 2;
}