Is it possible to change this to a better readable colour combination?
Either by the user or by the developer?
You can change the color of tags and nontranslatable fragments via the View > Colors menu.
Okay, you say: this is a user preference. Allright, I'll try to find a colour scheme that has more contrast for tab characters in purple. Not a problem.
Now I've set the color for non-translatables to 0, 254, 255 (RGB):
Which results in this nice and clear colour combo: