I am lost at this point. As you point out, the problem is in widget rendering specific to LO. There is certainly a setting somewhere. To make sure:
- test with a non-dark theme
- restart in safe mode (after backing up your user profile, just in case)
(… quick question, are we talking about LO’s or the OS/KDE theme there? I already tried changing the former from following the WM’s (which is Breeze (dark)) to a couple others, in particular, a freshly installed Oxygen, but that didn’t change anything beyond the icon artwork …)
… The “are you sure you want to restart in safe mode” popup is one of the non-resizeable and thus unusable ones …
… started with --safe-mode from the command line, problem persists (as witnessed by the huge selections/buttons in the first window, and "continue with safe mode"ing and opening the same spreadsheet) … 
