Would be perfect if it was able to customize current theme and/or themes grabbed on addons.mozilla.org...

I only downloaded this now, because the update yesterday (June 2021) Changed how everything looked, and I couldnt distinguish active tabs properly anymore. (everything either bright or dark)
I still liked it better before when all tabs had proper borders and didnt fade into each other, but with this I could at least make the background tabs dark blue and the active tab and bottom half with adress bar and bookmarks white, like it was before.

Unfortunately the Popup from bookmarks ignores the "popup background" setting and changes to simply "dark" if background tabs text is bright. I'd like this to be white with black text tho as well, like it's set up for popups in general. Please fix this.

In general though: Really nice add-on, easy to adjust and you see changes immediately.

It's a really great add-on.
Are you having trouble loading your theme collection?
The Firefox Color page should be reduced to about 80%.
Then you can load the saved themes.

I love making themes for myself and friends.