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!

It seems to work well, with good performance overall, but it indeed breaks some websites. That why it needs a exceptions list feature (if possible with smart segments of url, like: whitelisting .example.com' , 'example.com/subcategorypages' or "subdomainonly.example.com' ), and maybe for later, alternative methods to darken the pages (simple inverse filter, CSS) if the main one doesn't work as expected.
Also, it'd be cool to let the browser highlight with a different color visited links.

Encouragement to the dev!