Improve tree CSS

[jamesaoverton]

The list items are indented too much, which makes it hard to browse deep into the tree.

I would like to use CSS like:

• https://iedbtk.ontodev.com/search/?positive_assays_only=true&nonpeptide=CHEBI%3A33227
• https://github.com/jamesaoverton/iedbtk/blob/master/src/iedbtk/templates/base.jinja2#L28