style.css

html,
body,
#blocklyDiv {
  border: 0;
  margin: 0;
  padding: 0;
  width: 100%;
  height: 100%;
}

.tabs-level {
  height: 30px;
  margin-left: 10px;
  margin-top: 5px;
  padding-bottom: 5px;
  padding-inline-start: 0;
} 

.tabs-level li {
  font-family: sans-serif;
  display: inline-block;
  list-style: none;
  position: relative;
  overflow: hidden;
  height: 50px;
  width: 50px;
}

.tabs-level li:after {
  content: '';
  position: absolute;
  width: 300%; 
  height: 300%;
  top: 10px; 
  right: -40px;
  background-color: rgb(40, 40, 40);
  transform: rotate(45deg);
  z-index: -1;
}

.tabs-level li:not(:first-child) {
  margin-left: 4px;
}

.tabs-level li a,
.tabs-level li a:visited,
.tabs-level li a:active {
  color: rgb(138, 138, 138);
  text-decoration: none;
  position: absolute;
  text-align: center;
  width: 100%;
  height: 100%;
  line-height: 50px;
  transition: all .2s ease-in;
}

.tabs-level li.tab-active a {
  font-size: 25px;
  color: white;
}

.tabs-level li a img {
  filter: opacity(0.3);
  transition: all .2s ease-out;
  width: 26px;
  height: 26px;
  padding: 12px;
}

.tabs-level li.tab-active a img {
  filter: opacity(1.0);
  transform: scale(1.1);
}