style.css

html {
    box-sizing: border-box;
}

* {
    margin: 0px;
    padding: 0px;
    box-sizing: inherit;
    font-family: sans-serif;
}

body {
    background-color: powderblue;
    user-select: none;
}

body.defeat {
    background-color: rgb(240, 186, 157);
}

body.victory {
    background-color: rgb(158, 221, 143);
}

header, #message {
    margin-top: 50px;
    text-align: center;
}

#playfield {
    display: block;
    background: lightgrey;
    margin: auto;
    border: lightslategrey solid 1px;
    width: fit-content;
    margin-top: 50px;
    box-shadow: 0px 2px 6px rgb(0, 0, 0, 0.2);
    user-select: none;
    -webkit-user-select: none;
    -moz-user-select: none;
}

.cell {
    display: inline-block;
    width: 40px;
    height: 40px;
    border: rgb(110, 113, 116) solid 1px;
    text-align: center;
    vertical-align : middle;
    font-size: 16px;
    line-height: 40px;
    font-weight: 900;
    cursor: pointer;
}

.cell:hover:not(.discovered) {
    background-color: rgb(232, 232, 232);
}

.discovered {
    background-color: rgb(177, 177, 177);
}