Excellent add-on! Congrats and thank you to the Dev.

I tested all possible add-ons available for dark themes.
I also tested lot of dark theme scripts (Stylus and similar).
Also tested lot of CSS' scripts at userContent.css.
And this add-on is the best one among other alternatives. It has the best gain/loss equation (lightweight = low system impact, average good web-page layouts etc). In brief, it will work fine for most of the websites.

Sadly, this add-on has two week points:
1) At least 50% of web-pages have a white-flash when loaded, reloaded etc
2) On "Style-sheet-processor" option, at least 30% of web-pages break (for 1 second) at the very beginning when they start to load.

I found these two problems above almost in every similar add-on.
Only CSS' scripts and Stylus' scripts etc eliminate white-flashes + web-page broken on loading. It will be a dream if the Dev (Mikhail Khvoinitsky) can solve these two issues.

Thank you in advance!