style.css

body {
    background-image: url('cover.jpg');
    background-size: cover;
    background-repeat: no-repeat;
    max-width: 1920px;
}

h1 {
    text-align: center;
    color: azure;
    font-family: Verdana, Geneva, Tahoma, sans-serif;
    font-size: 60px;
    text-shadow: 2px 2px 5px rgba(0, 0, 0, 0.67);
}

.cal {
    background-color: rgba(104, 104, 104, 0.45);
    width: 380px;
    margin: auto;
    min-height: 450px;
    box-shadow: 0 8px 50px -7px black;
    border: 2px inset azure;
}

.calbody {
    display: table;
}

.display-box {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background-color: slategrey;
    border: none;
    color: white;
    width: 99%;
    height: 100px;
    font-size: 45px;
    text-align: right;
}

.calbody input[type=button] {
    width: 95px;
    display: table-cell;
    height: 87px;
    background: rgba(255, 255, 255, 0);
    border: 0.1px solid rgba(255, 255, 255, 0.301);
    color: white;
    font-family: Verdana, Geneva, Tahoma, sans-serif;
    font-size: 25px;
}

.calbody input:hover[type=button] {
    background: rgba(227, 227, 227, 0.30);
}

.calbody input:active[type=button] {
    background: rgba(55, 55, 55, 0.45);
}