I choose the user interface style as the notebookbar and for some reason the function color is white. I can’t see the function names (e.g. Copy), I can only see the empty boxes next to the icons.
Which version of LibreOffice (on which operating system) are you talking about (the phrase notebookbar for the user interface options doesn’t appear in more recent versions of LibreOffice)?
If set, try to disable OpenGL in `Tools -> Options -> LibreOffice -> View -> Category: Graphics Output -> Option: [ ] Use OpenGL for all rendering` and check whether that fixes your problem
LibreOffice 6.4.3.2 (x64). OS: Win10.
… and did you try the OpenGL thing?
You seem to use the dark theme (Tools -> Options -> LibreOffice -> Personalization -> [o] Preinstalled themes - Don’t use a theme and you get the default look (This got nothing to do with the UI type)