Something that bothers me almost every single time I use Krita is how I often have to waste time or go closer to the screen to find the scroll bar because it is only the slightest bit brighter than the rest of its 'track'. If I ever try to rush it, the view keeps jumping to places because i missed the actual bar. Even trying out different themes didn't fix this. Anyway, unless there's already a solution to this that I don't know of, or unless it's already been fixed and I just haven't updated yet, please make the contrast with the scrollbar higher.

You can make your own color themes as well, but apart from changing the theme colors, there's nothing we can do about the scrollbar itself. We don't create our own widget theme -- but you can try some of the default styles by starting krita on the command line like this:

KRITA_NO_STYLE_OVERRIDE=ON /path/to/krita --style Windows

(The exact command depends, of course, on your OS)

ETA: depending on your OS, other style options are plastique, Fusion, Breeze, Oxygen, QtCurve or Windows.