I have edited documents for decades and the black text on white background is the worst choice for your eyes. In LibreOffice Preferences, I am able to change the background color without a problem, but the font color preference will not take effect. LO v. 6.2.2.2. running on MacOS Mojave. Am I missing something?
You’re oviously on a Mac, but you also forgot to mention LO version.
I’ve got Version: 6.2.1.3 (Build ID: e485074f4984fbc86184b6aa3aba3228d0150755) and this setting works just fine for me. I’m still on Yosemite, though (10.10.5).