The add-on suddenly went into dark mode with FF96. I reverted to FF95 to test the issue and found that the theme worked properly until FF96 was re-installed. This seems to be an issue with FF96 and the way it interacts with Dark Fox. Note that Dark Fox has not been updated for quite a while (May 2019) so it is the update to FF that broke the theme. Perhaps the developer can look into the issue and resolve it so that Dark Fox works as it did prior to FF96. Until then there are some workarounds on the web that partially correct the problem. See, e.g. https://gist.github.com/gmolveau/a802ded1320a7591a289fb7abd0d6c45 (note: change the rgb colors to 255,255,255 and the background color to #fffff in the css files). This in combination with the suggestion below ( in about:config--"layout.css.prefers-color-scheme.content-override" from 3 to 2 ) helps further