style.css

* {
    box-sizing: border-box;
    font-family: 'Syne Mono', monospace, sans-serif;
}

body {
    margin-bottom: 100px;
}

#main {
    display: flex;
    align-items: center;
    justify-content: center;
}
.main-header {
    text-align: center;
    margin-top: -12px;
}

.calculator {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    border: 5px solid #ccc;
    border-radius: 20px;
    width: 300px;
}

#calcDisplay {
    width: 100%;
    font-size: 45px;
    height: 75px;
    border-radius: 15px;
    background-color: #252525;
    color: #ffffff;
    text-align: right;
    padding-right: 20px;
    padding-left: 10px;
}

.calc-keys {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    grid-gap: 8px;
    padding: 8px;
    
}

.button {
    height: 60px;
    background-color: #fff;
    border-radius: 10px;
    border: 1px solid #c4c4c4;
    background-color: transparent;
    font-size: 25px;
    color: #000000;
    box-shadow: 0 0 3px rgb(83, 83, 83);
    margin: 2px;
    padding: 10px;
}
.button.operator {
    background-color: #2fb9f0;
    border-color: #337cac;
}
.button.action {
    color: #f0595f;
}
.button:hover {
    background-color: #eaeaea;
    font-weight: bold;
}
#clearBtn {
    background-color: #f0595f;
    color: #000000;
}
#clearBtn:hover {
    background-color: #eaeaea;
    color: #f0595f;
    border-color: #c0367b;
}
#equalBtn {
    grid-column: span 2;
}
#clearBtn {
    grid-column: span 2;
}

.footer {
    position: absolute;
    bottom: 0;
    left: 0;
    display: flex;
    align-items: center;
    justify-content: space-around;
    width: 100%;
    font-size: 10px;
}