html {
    width: 100%;
    height: 100%;
    box-shadow: inset 0 0 100px black;
}
body {
    background: orange;
    color: black;
    transition: background, color;
    transition-duration: 0.5s;
}
#main {
    width: 540px;
    height: 600px;
    margin: auto;
}
#buttons {
    padding-top: 0;
    margin-top: 0;
    width: 540px;
    height: 540px;
    display: grid;
    grid-template-rows: repeat(3, 180px);
    grid-template-columns: repeat(3, 180px);
}
#new {
    font-family: sans-serif;
    width: 540px;
    height: 60px;
    background: lightblue;
}
#a1, #a2, #a3, #b1, #b2, #b3, #c1, #c2, #c3 {
    background: lightblue;
    font-family: sans-serif;
    font-size: 72;
}
h1 {
    font-family: sans-serif;
    text-align: center;
    padding: 0;
    margin: 0;
}