main.css

body{
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
    background-color: aquamarine;
}

h1{
    display: block;
    font-size: 40px;
}

table {
    border-collapse: collapse;
    font-family: Calibri, sans-serif;
}

colgroup, tbody {
    border: 4px solid black;
}

td {
    border: 2px solid black;
    height: 2em;
    width: 2em;
    text-align: center;
    padding: 0;
}

button.solve{
    font-size: 20px;
    margin: 1%;
}

table.keypad{
    visibility: hidden;
    font-weight: bold;
    width: 15%;
    height: 38%;
    background-color: blueviolet;
    position: absolute;
    top: 25%;
    left: 42.5%;
    opacity: 0.98;
}

@media (max-device-aspect-ratio: 1/1) {
    table.keypad{
        left: 30%;
        width: 40%;
    }
}


table.keypad td{
    border: 2px solid black;
    font-size: 120%;
}

table.keypad td:hover{
    background-color: yellow;
}