Using light or dark colour schemes is information, that websites can gather on you. Therefore this addon is obviously incompatible with anti fingerprinting. You can't have it both ways. If you disable the fingerprinting protection you'll find the addon works as described.

Note that this is explicitly specified in the add-on description.

Are you sure you are checking on website or pages that support dark mode? (Try for example on about:blank or duckduckgo.com .) Note that if you have set privacy.resistFingerprinting to true, Firefox also prevents the add-on from working, as specified in the add-on description.