The default font colors used with conditional formatting are not readable due to awful contrast.
The options menu allows only to change the background, not the foreground?
Am I missing something here, will it really use yellow background with white text ?
The Automatic font colour appears to refer to the background colour of the cell, not the conditional formatted background colour. You should report a bug, How to Report Bugs in LibreOffice - The Document Foundation Wiki
Workaround
Create a cell style for your data and set Font Effects > Font colour to black there. In the example attached, the cell style Data is applied to cells A2:A31.
ConditionalAllCellsDarkMode.ods (18.8 KB)
Another option is to pick colors that contrast with white, which could be darker variants of the same hues.
It’s a shame it doesn’t handle the dark mode case particularly well. I did check, and if I turn off dark mode, the foreground color updates to black, and everything works properly.
I’ll create a bug report and report back here.
