Simple Black X will be the complement to Simple White X, which is a revision of Simple White by Louis Chan. Currently this is just an anouncement of what is to come here. The tag line for the theme is well known:
Black is beautiful.
The theme shall be close to the inverse of Simple White X, but this concept may only apply to monochromatic (black/white/gray) aspects of the theme. Anything with color should keep the same hue, changing the lightness and saturation if needed. The preparation of this theme needs special attention to the identity box icons and the website identity icons (fav icons), which may need a special background, probably a gradient. The same attention has to be done to the making of the tabs bar.
Please be mindful this work is at a very eary planning stage, so this will be mostly empty for a while. The bulk of it will come when it is ready. I am yet busy with work for Simple White X because the next release date is close while I need to correct a few things for so Firefox 45 soon to be Developer Edition (and future ESR).
Suggestions are welcome and I shall take them into consideration.
I am unclear of the future of this project given @rapisho published the theme Simple Black at AMO. This theme appears to take the code of Simple White X and change every color to its RGB inverse, e.g., change #123def to #edc210. This is a bit different to what I had in mind, because that way blue turns into yellow. But actually I see that in the theme most of the blue is changed into orange, so perhaps it is not a simple color inverse. The toolbar icons and a few others are also a bit different. The version I found was at an early stage. but I liked overall result: a cool light theme is changed into a dark warm theme.