I updated to the most recent version of LibreOffice (26.2.1) and it reset my background colors. Writer let me change it fine, but with Calc, no matter what I try, the color will not change. I’ve seen other people mention that disabling high contrast in the accessibility settings made it work for them, but that did nothing for me. Literally nothing is working, and I cannot function with a white background.
I assume that you are using dark mode. Click Tools - Options - LibreOffice - Appearance and untick Use white background for document or words to that effect.
Thank you so much, that did the trick!