I like the feature, and it's fun to use. However, it's not clear how to make the theme active. It doesn't appear in the list of themes in settings, and clicking on it in the page where I saved it does nothing. Saving and using should be intuitive, and the theme should show up in settings for ease of use.

It doesn't work on changing theme

Small print and "fine" and small print/fonts, as in the top list of browsers, are a strain for my aging eyes.

Really nice, but it's missing the option to select colors (text and background) for bookmarks-bar, when opening a bookmark-folder there. - On my system the popup-list is black with white text, but i would prefer at least the opposite and cannot change it anywhere. :/

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.