Such a huge and depressing step backwards from Personas.
The tiny array of options it allows all work perfectly, and while it is probably far simpler to use that Personas for creating a theme, you could equally argue that MS-Paint is simpler to use than Photoshop - that doesn't make something fit for purpose.

I can not register my theme online because I put a personal image; complicated because I have to remove the extension and put it back on and I have to start all over again.

Would be great as I easily created my own custom theme (Spider-Gwen) but now my last created settings in Firefox Color are set as my theme every time Firefox fully initializes after closing overriding my current theme selection, which I then have to go into the settings to deactivate my theme, then reactivate it to overwrite what Firefox now thinks is my default theme from Firefox Color.

The add-on offers very comprehensive customization, as far as controlling colours of things goes. However, throughout my usage I've noticed two flaws:
1. The default about:newtab page colour is forced to a white background with any theme. If the add-on is going to change it, then that should be part of the customization.
2. Clicking the add-on icon will automatically turn on a theme. The add-on shouldn't do this, instead doing nothing until a user saves the theme.

GARBAGE! What could Mozilla possibly be thinking?? Set-up is a pain, unless you choose to use one of the "pre-designed" themes that's offered. Trying to get text to display correctly in the search bar, bookmark bar, or side bar is often hit or miss. Most annoying of all though is that if you do find a "theme" that you can TOLERATE, Firefox sync does NOT install it to all of your computers automatically, leaving one to have to do this on EVERY SINGLE MACHINE individually! I'll allow two stars because Firefox is still my browser of choice on my Linux computers, but as far as theming is concerned, Mozilla has taken a HUGE step backwards!!