body {
   font-family: Arial, Helvetica, sans-serif;
}

section {
   margin-left: auto;
   margin-right: auto;
   width: 430px;
   text-align: center;
}

#cardGrid {
   font-size: 75px;
   display: grid;
   grid-template-columns: 100px 100px 100px 100px;
   grid-template-rows: 100px 100px 100px;
   grid-gap: 10px;
}

#cardGrid > div {
   border: 2px solid steelblue;
   border-radius: 15px;
   background-color: steelblue;
   text-align: center;
   line-height: 100px;
}

#cardGrid > div:hover {
   border-color: red;
}

#cardGrid > .card-visible {
   background-color: white;
}

#newGameBtn {
   margin-top: 25px;
}