.reference-book {
  .toc {
    max-height: 100%;
    overflow-y: scroll;
    a {
      #fonts > .sans();
      color: @tutor-black;
        display: block;
        &:hover {
          background-color: @tutor-neutral-bright;
          text-decoration: initial;
        }
        &.active {
          background-color: @tutor-neutral-light;
          text-decoration: initial;
        }
    }
    li {
      list-style-type: none;
      #fonts > .sans(1.3rem);
      border-top: 1px dotted @tutor-neutral;
      a {
        color: @tutor-black;
        padding: 5px;
      }
    }
    ul { padding-left: 0rem; }
    
    ul[data-depth='1'] a{
      #fonts > .sans(1.5rem);
      padding-left: 1rem;
    }
    ul[data-depth='2'] a{
      #fonts > .sans(1.4rem);
      padding-left: 2rem;
    }
    ul[data-depth='3'] a{
      #fonts > .sans(1.3rem);
      padding-left: 3rem;
    }
    ul[data-depth='4'] a{
      #fonts > .sans(1.2rem);
      padding-left: 4rem;
    }
  }
  .content {
    .toc {
      li { list-style-type: none; }
    }
  }
}

.reference-book {
  .toc-indented-levels(@depth) when (@depth > 0) {
    .toc-indented-levels((@depth - 1));
    ul[data-depth='@{depth}'] a{
      #fonts > .sans( unit((1.6 - (@depth / 10)),rem) );
      padding-left: unit(@depth, rem);
    }
  }
  .toc {
    max-height: 100%;
    overflow-y: scroll;
    a {
      #fonts > .sans();
      color: @tutor-black;
        display: block;
        &:hover {
          background-color: @tutor-neutral-bright;
          text-decoration: initial;
        }
        &.active {
          background-color: @tutor-neutral-light;
          text-decoration: initial;
        }
    }
    li {
      list-style-type: none;
      #fonts > .sans(1.3rem);
      border-top: 1px dotted @tutor-neutral;
      a {
        color: @tutor-black;
        padding: 5px;
      }
    }
    ul { padding-left: 0rem; }
    .toc-indented-levels(4);
  }
  .content {
    .toc {
      li { list-style-type: none; }
    }
  }
}

Use looping mixin for stying the TOC levels
Use looping mixin for stying the TOC levels
