I had never thought about using the Stylish app (an extension for Firefox, Chrome, etc., see the respective stores) to modify how Roll20 displays. It turns out that people have created many such stylesheets! I’ve just found two that are quite useful to me.
One lets you add rows to the map tab, i.e., the list of pages (maps) wraps instead of extending off to the right of the visible area:
The other creates tighter sidebar lists in the Art Library and Journal tabs:
The important thing to remember is that in all cases, the modifications will only be visible to the person using the stylesheet. So if you pick one of the many pretty themes, you’re the only one benefiting from it unless others in your group also add the Stylish extension and this specific stylesheet.
In some cases a stylesheet can interfere with a particular character sheet or API. In that case, it’s easy to disable the stylesheet in the app controls.