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.