toc.css

@-webkit-keyframes fadeIn {
  from { opacity: 0; }
    to { opacity: 1; }
}  
@keyframes fadeIn {
  from { opacity: 0; }
    to { opacity: 1; }
}

#TOC {
  width: 20rem;
}

#TOC::before {
  content: "☰ Contents";
  font-size: 1.7rem;
  font-variant: small-caps;
  cursor: pointer;
  display: inline-block;
  padding-left: 10px;
  width: calc(20rem - 10px);
  border-radius: 5px 5px 0 0;
  border-bottom: 2px double #fefefe;
}

#TOC:hover {
  color: white;
}

#TOC:hover:before {
  background: #2c2c2c;
  border-bottom: 2px double #4f4f4f;
  -webkit-animation: fadeIn .5s;
  animation: fadeIn .5s;
}

#TOC ul {
  padding: 0;
  margin: 0;
  display: none;
  position: absolute;
  font-size: 1.2rem;
}

#TOC:hover > ul {
  display: block;
  z-index: 1;
  -webkit-animation: fadeIn .5s;
  animation: fadeIn .5s;
}

#TOC li {
  padding: 0 0 0 0;
  position: relative;
  display: block;
  width: 20rem;
}

#TOC > ul > li {
  border-bottom: 1px solid #4f4f4f;
}

#TOC ul ul {
  position: absolute;
  display: none;
  left: 100%;
  top: 0;
}

#TOC li:hover ul {
  width: fit-content;
  display: block;
  -webkit-animation: fadeIn .5s;
  animation: fadeIn .5s;
}

#TOC li:hover > ul {
  display: block;
}

#TOC a, .part, .appendix {
  background: #2c2c2c; /*linear-gradient(to bottom, #2c2c2c 0%, #141414 100%);*/
  color: #ffffff;
  display: block;
  padding: 10px;
  text-decoration: none;
  text-shadow: none;
}

#TOC .part, .appendix {
  background: #0fa1e0;
  text-align: center;
  font-size: 1.2rem;
}

#TOC > ul > li > a:hover{
  background: #1e1e1e;
}

#TOC .has-sub ul li a {
  background: #0fa1e0;
  border-bottom: 1px dotted #31b7f1;
  filter: none;
  display: block;
  line-height: 120%;
  padding: 10px;
  color: #ffffff;
}

#TOC .has-sub ul li:first-child a {
  border-radius: 0 5px 0 0;
}

#TOC .has-sub ul li:last-child a {
  border-radius: 0 0 5px 0;
}

#TOC .has-sub ul li:hover a {
  background: #0c7fb0;
}

#TOC ul ul li:hover > a {
  color: #ffffff;
}