Several deficiencies & bugs

creating a export XPI or zip file of the theme cannot be imported or installed in other Firefox browsers.
Under themes > gear symbol> "install add-on from file..."
selecting either an XPI or zip file of the theme created and exported creates the following error result:

about:addons
This add-on could not be installed because it appears to be
corrupt.

FirefoxColor and the saved custom theme should be incorporated with the add-ons so that syncing across Firefox browsers the theme also gets synchronized.
The setup and configuration of the FirefoxColor interface is a bit crude and clunky lacking more details what settings/colors changes what & where. Furthermore, on the saved themes it does not become clear when there are more than one theme saved, which one is the last one. Neither does the crude visual representations indicate any subtle changes e.g. between 2 themes.

The generated xpi file is not signed because it is generated locally. You can try it out by loading it temporarily at about:debugging. To use it persistently, the package needs to be signed, by submitting it to addons.mozilla.org.
Sounds reasonable. This could technically be implemented via the storage.sync API, but the downside is that the storage quota is limited, which may result in broken themes if (large) images are chosen as a background.
The page features a preview, and when saved the browser is updated. How can the user experience be improved according to you?

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