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.