There is an issue Using Any Dark Theme on Websites that dont allow Dark Theme

[GeckoPixel]

Some pages from the G-suite that doesnt have an option for Dark Theme, if using any Dark Theme will have some weird behavior with the background of this sites

This issue could be prevented changing the user_pref("browser.tabs.allow_transparent_browser", true); from true to false, because is forcing the background to be transparent and use the color of the Theme, but it wont change the color of the text so its hard to read.

[Godiesc]

hi,
Setting false the preference browser.tabs.allow_transparent_browser should solve any related issue, if not maybe is an issue with the page itself or other extension. You have here other solution: #272 and #298