Since Firefox so diligently created incompatibilities one after another one and with every update of the endless updates, although not perfect, but this is a very welcome add-on! Let's see how long it takes until Firefox manages to make it incompatible again!

es hermoso!!!

This is, as far as I can tell, completely useless. Managed to get it to display a custom image, eventually and more by fluke than anything. The tab for the saved "themes" is hidden by the text box on smaller screens so didn't even find it until by chance looking on another computer. The main reason I wanted to customise the theme was so I could distinguish between personal/work profiles, but the "theme" doesn't sync when using Firefox Sync so this is not only unusable but also useless. Do not see the point of this add-on.
Also, minor annoyance, from the settings page there is a link that has been localised to say "Firefox Colour" but it is still called "Firefox Color" everywhere else. Pointless localisation there, either do it everywhere or nowhere.

One of the most CREATIVE intelliegient choices we have to surf the web safely. Security is a major issue these days. What would we do without Developers like the MOZILLA team?
The first major browser that developed a platform for ANYBODY to use "COLORS "to really customize their experience, regardless of webdesign experience.
Chrome is getting better, Brave seems to be SNOBBY & Edge has a long way to go with giving the user full control!

Causes conflicts with Firefox's themes. Only use one or the other.

ブラウザーの色を変えられて、好きな色で作業ができるのはいいが日本語に対応していないので、使い方側時からないところがあった

I just wanted to change my top bar colour to blue and it's so complicated? Other colours randomly changed on other UI elements that I didn't ask for? Could you have made this any more baffling?
I was also turned off by Firefox marketing associating "blue" with "activist". Yeh no thanks pal, I just like the colour, save me the cod psychology...