Open up about:config and go to layout.css.prefers-color-scheme.content-override and set it to one of the following:
0 => Force a Dark background
1 => Force a Light background
2 => Follow the System theme (default in Firefox 94)
3 => Follow the Browser toolbar theme (default in Firefox 95+)
I have astigmatism and disagree wholeheartedly. Light mode is like staring into the sun.
Just looks like this for me
Just need a well lit room for light mode and at least the white of the background bleeds into black text making it grey rather than the blurry mess of white on black.