I fully agree with what @HarSel has to say about lack of contrast, and this issue has been discussed before: Themes too difficult - #17 by former_evernotist
Unfortunately, CSS tweaking is no easy fix and can cause issues when Joplin is updated (which fortunately happens quite often).
I also agree with HarSel's opinion that scroll bars are to small in width: In my humble opinion those narrow bars (and even those 'dynamic' ones that get wider when you use them) belong to a widespread, but annoying fashion of UX design...