A recent update has essentially broken multiple sites for me. Firefox has the following dark/light mode settings: Automatic, Dark, Light.
If I set it to Automatic, and my desktop is in dark mode, it now tries to apply dark mode to every site. Unfortunately many sites are just broken, notably Outlook (black text on dark grey background -- interestingly it doesn't always load this way, only about 90% of the time).
However, if I switch to Light mode, now my entire browser is in light mode, which I don't want.
Firefox either needs separate light/dark mode settings for sites vs the browser chrome itself, or site-specific settings so I can set specific ones to use lightmode.
I find the entire concept of forcing dark mode on websites that are specifically not supporting it to be completely ridiculous.
I believe I have the same issue too. I can't find a solution to it after searching for a little bit. I have attached a screen shot of a webpage that has been broken by my issue. I am unsure if this is the similar or the same to your issue but from what you describe, I think it is. The site I used for this test. If you visit this site on Chrome, it looks fine. Visit it on Firefox(at least for me) and the colors of the page are messed up.