Cell backgrounds are invisible.
The cell has the background property, because when I double-click it to insert a value I can see the background. When I load Print Preview it also shows correctly.
Also when I copy and paste the cell to a different spreadsheet editor (e.g. Google Sheets) the cell background does show up there correctly. Only LibreOffice doesn’t render it.
I tried changing document background, setting and resetting the different options regarding system colors and high contrast compatibility / openGL
Also tried safe mode and resetting user profile.
Themes are broken so can’t test that.
The cell backgrounds do show up as they should when switching off Windows high contrast theme.
Using LibreOffice 18.104.22.168 on Windows 10 x64.