style.css

@font-face {
  font-family: "Cutive Mono";
  src: url("CutiveMono-Regular.ttf") format("truetype");
}

body {
  --highlight-color: #f57c00; /* Orange 700 */
  align-items: center;
  background: #212121; /* Grey 900 */
  display: flex;
  font-family: "Cutive Mono", monospace;
  font-weight: 400;
  height: 100vh;
  justify-content: center;
  overflow: hidden;
  transition: 0.4s;
  width: 100vw;
}

.clock {
  display: flex;
  flex-flow: column wrap;
  height: 100px;
  position: relative;
  width: 280px;
}

.block {
  color: #424242; /* Grey 800 */
  height: 20%;
  transition: 0.4s;
  width: calc(100% / 12);
}
.block.active:before {
  color: #eeeeee; /* Grey 200 */
  font-weight: 700;
}
.block.highlight:before {
  color: #f57c00; /* Orange 700 */
  color: var(--highlight-color);
}
.block:not(:nth-child(n + 16)):nth-child(n + 11),
.block:not(:nth-child(n + 46)):nth-child(n + 41) {
  margin-right: 10px;
}
.block:not(:nth-child(n + 31)):nth-child(n + 26) {
  margin-right: 20px;
}
.block:nth-child(-n + 10):before {
  content: "0" attr(data-num);
}
.block:before {
  content: attr(data-num);
  display: block;
  font-size: 16px;
  height: 100%;
  line-height: 20px;
  position: relative;
  text-align: center;
  width: 100%;
}

.divider {
  background: #424242; /* Grey 800 */
  height: 60%;
  left: calc(50% + 19px);
  position: absolute;
  top: 20%;
  width: 2px;
}

body.light-theme {
  background: #e0e0e0; /* Grey 300 */
}
body.light-theme .block {
  color: #bdbdbd; /* Grey 400 */
}
body.light-theme .block.active:before {
  color: #424242; /* Grey 800 */
}
body.light-theme .block.highlight:before {
  color: #f57c00; /* Orange 700 */
  color: var(--highlight-color);
}
body.light-theme .divider {
  background: #bdbdbd; /* Grey 400 */
}