UltimaDark has the potential to be the best dark-theme add-on (ever). I'm using the word "potential" just because now UltimaDark is in "experimental stage". But the Dev is very active and responsive, and he is working very hard to improve the add-on, and to add new features. Progress can be followed at Dev' GitHub.

Yesterday the ON/OFF was included (among other improvements). And white-listing is already on the way. Other features are planned, and surely with time we're going to see great new stuff.

Most of the webpages work great with this add-on. And only few webpages have issues (but currently the Dev is working on it, trying to fix everything). It's worth mentioning that is impossible to build the perfect dark-theme add-on. Lot of webpages are badly programmed, and it's not easy to deal with them. Every dark-theme add-on has imperfections.

IMHO UltimaDark is great firstly because its browser performance (webpages have a good loading speed). And secondly, this add-on has very nice aesthetic results. In brief, this add-on is great for the relation: "Browser Performance" VS "Aesthetic results".

Thank you and congratulations to the Dev!