That screenshot only shows one of the configurations I’ve tried. I’ve since uninstalled and reinstalled LibreOffice twice. I don’t know how much more of a reset I can do.
Can you make a new document look ok in safe mode? Either dark or light mode?
You could reset your user-profile. This is where your settings are stored. The profile is not touched by updates/re-install, as you usually don’t wish to loose all settings on updates.
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As you already tried safe mode this may not solve your problems, but may bring you back to the level before you started your experiment.
I followed these instructions, but nothing changed. I’m still getting a black background with black text. I don’t know what to do now. I can’t see the text on my documents.
Might be Bug 154916 - Switching Dark | Light or vice versa in View > Appearance and Applying results in wrong colors in Preferences Dialog
After resetting user profile change nothing in user personalisation except to change Automatic field to System Theme, Apply and OK out. Restart (probably LibreOffice but maybe Mac as well, tdf#156855 is not clear on this point). After restart, don’t check with your existing document, but create a new document using default settings, paste special some unformatted text and see if it looks OK.