Janmarc,
A little more explanation of your images and details may help. TW5 Version, any particular themes, or dark mode active etc...
I see in the image "#ffffff!importiant;" I would search for this if possible in the wiki. Even in a text editor if unreadable in the browser.
They say "avoid the use of the CSS important at all costs", perhaps this is an example of why.
If you have a custom stylesheet or two disable them by removing the stylesheet tag. backup, but you can search for the tag and rename it in the text file on custom tiddlers.
There is more we can help with but way more info can come from you.
Tones