Dear community,
After changing the icon-theme from automatic to sifr (dark) all conditionally/automatically or manually set background colours are not shown in LibreOffice Calc. If the document is exported to PDF the colours are displayed correctly.
After reinstalling LibreOffice the problem still persisted.
Your support is greatly appreciated!
How did you change it? If it was by clicking Tools - Options - LibreOffice - Accessibility then setting High Contrast to Enable, or by setting the operating system to High Contrast with the default setting in LibreOffice unchanged, then that will turn off colour visibility.
Set LibreOffice High Contrast to Disable, in Tools - Options - LibreOffice - View set icon to Sifr (dark).