style.css

:root {
  --bkgr: #eee;
  --text: hsl(0, 50%, 50%);
  --border: hsl(40, 100%, 27%);
}

* {
  box-sizing: border-box;
}

body {
  background: var(--bkgr);
  font-family: 'Open Sans', Arial, 'Helvetica Neue', Helvetica, sans-serif;
}

h1 {
  color: var(--text);
  font-family: Georgia, 'Times New Roman', Times, serif;
  font-size: 48px;
  margin-bottom: 0;
  text-align: center;
}

div#score-holder {
  color: var(--text);
  font-size: 32px;
  float: right;
  width: 7em;
}

#game-grid {
  clear: both;
  margin: 100px auto 0 auto;
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  grid-gap: 2px;
  position: relative;
  width: 500px;
}

.game-cell {
  align-items: center;
  border: 2px solid var(--border);
  border-radius: 10px;
  display: grid;
  font-size: 64px;
  height: 120px;
  justify-content: center;
  width: 120px;
}

#result {
  border: 3px solid black;
  border-radius: 12px;
  display: none;
  height: 400px;
  left: 40px;
  padding-top: 90px;
  font-size: 72px;
  position: absolute;
  text-align: center;
  top: 110px;
  width: 540px;
  z-index: 2;
}

#result button {
  background: white;
  border: 2px solid black;
  border-radius: 8px;
  color: #008;
  font-size: 20px;
  margin: 40px auto;
  padding: 10px 0;
  width: 12em;
  z-index: 3;
}