In Writer I get all onscreen help dialogues with black characters on a nearly black background; impossible to read! Can’t find any clue and didn’t make any changes to the original install.
I use Kubuntu and have all official updates.
Seems like LibreOffice is wrongly detecting your theme’s colors - using a different desktop-theme might help.
You can also force LibreOffice to not use the system’s themeing: try to export SAL_USE_VCLPLUGIN=gen (for generic) or OOO_FORCE_DESKTOP=none
If any of the above helps, you should file a bug against the KDE-theme you’re using.