index.css

/* li.selected {
  border-bottom: 4px solid rgb(58, 127, 255);
} */

.the-menu {
  position: relative;
  font-family: "Gill Sans", "Gill Sans MT", Calibri, "Trebuchet MS", sans-serif;
  font-size: 24px;
  margin: 50px auto 0;

  display: flex;
  justify-content: center;
}
.the-menu > ul {
  display: flex;
  margin: 0;
  padding: 0;
}
.the-menu > ul > li {
  display: block;
  margin-left: 40px;
  cursor: pointer;
  opacity: 0.3;
  font-weight: bold;
  transition: opacity 0.2s;
  padding-bottom: 10px;
}
.the-menu > ul > li:first-child {
  margin-left: 0;
}
.the-menu > ul > li:hover,
.the-menu > ul > li.selected {
  opacity: 1;
}

.the-marker {
  display: block;
  position: absolute;
  height: 4px;
  background-color: rgb(58, 127, 255);
  left: 0;
  transition: all 0.4s;
}

.the-marker.off {
  transition: none;
}

button {
  padding: 6px 16px;
  background: rgb(22, 80, 187);
  color: #fff;
  font-size: 16px;
  border: none;
  border-radius: 5px;
  outline: none;
  cursor: pointer;
  transition: all 0.2s;
  box-shadow: 2px 2px 6px rgba(0, 0, 0, 0.4);
}

button:hover,
button:active {
  background-color: rgb(12, 54, 133);
}
button:active {
  box-shadow: none;
}

.actions {
  margin-top: 50px;
  display: flex;
  justify-content: center;
}

.actions > button {
  margin-left: 20px;
}
.actions > button:first-child {
  margin-left: 0;
}