I've been using this program for over a decade. I think the theme editing features need major work. It is very limited the way it is. Which is a little funny to say since if you look at the theme editor options, it sure looks like there are a ton of features and any little thing can be tweaked. But not really.
While some things can be very heavily customized like the underline style and highlighting etc., there are several very critical things that cannot be modified, and if they can't be modified, all these other features are almost useless as they will not look good with the ones you cannot change. let me explain...
the mouse-over highlight...
When you mouse over stuff, it turns some kind of white-ish color. This works for default theme well. But if you want any kind of dark theme, this will ruin it. If you make the items dark wherever you can, this highlight wont change, and you will be left with a somewhat dark theme with an unusable mouse over highlight.
this is a big one. I've waited version after version, it never changes. I bet it's hard to change the way its developed. I've brought this up years ago also somewhere.
you cant change the bright green selected color in the Views sidebar. It will always be green.
you can't change the striped progress bar that appears across an item if it is tagged as a "project" it will always be grey-white
there are probably others.