body {
  font-family: monospace;
  text-align: left;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  margin: 0;
  background-color: darkgreen;
  overflow: hidden;
  position: relative;
}
.container {
  font-size: 1.5em;
  width: 80%;
  max-width: 600px;
  line-height: 1.6;
  padding: 1em;

}

.letter {
  transition: background-color 0.3s, color 0.3s;
  color:crimson;
  background-color:white
}
.highlight {
  background-color: crimson;
  color:white
}