style.css

body {
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background-color: #eee;
}

.calculator {
    --button-width: 80px;
    --button-height: 80px;
    display: grid;
    grid-template-areas: "result result result result" "ac plus-minus percent divide" "number-7 number-8 number-9 multiply" "number-4 number-5 number-6 subtract" "number-1 number-2 number-3 add" "number-0 number-0 dot equal";
    grid-template-columns: repeat(4, var(--button-width));
    grid-template-rows: repeat(6, var(--button-height));
    box-shadow: -8px -8px 16px -10px rgba(255, 255, 255, 1), 8px 8px 16px -10px rgba(0, 0, 0, .15);
    padding: 24px;
    border-radius: 20px;
}

.calculator button {
    margin: 8px;
    padding: 0;
    border: 0;
    display: block;
    outline: none;
    border-radius: calc(var(--button-height) / 2);
    font-size: 24px;
    font-family: Helvetica;
    font-weight: normal;
    color: #999;
    background: linear-gradient(135deg, rgba(230, 230, 230, 1) 0%, rgba(246, 246, 246, 1) 100%);
    box-shadow: -4px -4px 10px -8px rgba(255, 255, 255, 1), 4px 4px 10px -8px rgba(0, 0, 0, .3);
}

.calculator button:active {
    box-shadow: -4px -4px 10px -8px rgba(255, 255, 255, 1) inset, 4px 4px 10px -8px rgba(0, 0, 0, .3) inset;
}

.result {
    text-align: right;
    line-height: var(--button-height);
    font-size: 48px;
    font-family: Helvetica;
    padding: 0 20px;
    color: #666;
    overflow: hidden;
}