Hello @Craf, welcome to the Nextcloud community.
you posted twice 
and didn’t fill the required support template… but honestly I don’t think it would help in this case.
It looks the dark theme is somewhat unreadable. From my testing pdf viewer and office follow system setting for dark theme. In my case pdf itself looks good but I see similar issue in the menu bar
in office it is more or less opposite - menu bar is good but some documents look weird
others even more:
nice catch from collabora forum - choosing “automatic” color makes text readable again (same doc as in the 1st screenshot):
for Nextcloud I see this Github issue:
it doesn’t exactly fit my testing - for me the files, photos, activity, settings, dashboard apps have perfect contrast (on desktop nc31), unreadable icons only appear in pdf reader. but I’m too lazy to search further - you should have enough trails to follow