It is very easy to use for making themes. It has a color palette which lets you use colors that are already used, and there is an advanced mode to color specific parts of the browser, complete with a manual explaining where each color goes.

However, it has issues:

1. As of the latest Firefox update, there are some advanced colors that don't work anymore, such as the tab background separator, pop up highlight and the new tab text.

2. There are also new attributes in the new Firefox version that can't be edited yet, like the border around the text in the ≡ menu which appears when using up/down arrows.

3. There is currently no way to navigate to saved ♡ themes.

There is also room for improvement:

1. An option to upload a theme file (.xpi) to edit

2. An option to edit the Firefox default themes

All in all, good app.
Hopefully the developers update it soon.