hydra/src/root/static/css/tree.css

ul.tree, ul.subtree {
    padding: 0;
    margin: 0;
    list-style-type: none;
}

/* Indent each tree level and draw vertical tree lines next to all
   children except the last. */
ul.subtree > li {
    position: relative;
    padding-left: 2.0em;
    border-left: 0.1em solid #6185a0;
}

ul.subtree > li:last-child {
    padding-left: 2.1em; /* for the 0.1em border-left in the last child */
    border-left: 0;
}

/* Draw the vertical tree line to the last child. */
ul.subtree > li:last-child > *:first-child:before {
    border-left: 0.1em solid #6185a0;
}

/* Draw horizontal tree lines to all children. */
ul.subtree > li > *:first-child:before {
    content: "";
    position: absolute;
    height: 0.65em;
    left: 0em;
    width: 1.5em;
    border-bottom: 0.1em solid #6185a0;
}

/* Ensure that the + and - have the same size. */
a.tree-toggle {
    text-decoration: none;
    font-family: monospace;
}