index.css

body {
    background-color: black
}
.container {
    margin-top: 10vh;
    text-align: center
}
.empty {
    margin: 30px;
    width: 150px;
    height: 150px;
    border : 2px solid white;
    border-radius: 5px;
    display: inline-block;
}
.hovered{
    -webkit-box-shadow: inset 0px 0px 20px 2px rgba(13,120,120,1);
-moz-box-shadow: inset 0px 0px 20px 2px rgba(13,120,120,1);
box-shadow: inset 0px 0px 20px 2px rgba(13,120,120,1);
}
h1{
    margin: 100px auto;
    text-align: center;
    color: blue;
    font-family: monospace;
    font-size: 45px
}
p{
    color: white;
    text-align: center;
    margin: 42% auto;
}

.filled{
    position: absolute;
    margin: 4px;
    width: 140px;
    height: 140px;
    cursor: pointer;
    text-align: center;
    color: greenyellow;
    border: 2px dotted greenyellow;
}
.invisible{
    display: none
}
h5{ 
    color: red;
}