This is a good add-on. It works exactly how you think it should.

One thing is that the preferences UI is arranged vertically, not horizontally. There's a lot of extra space that doesn't get used in the main window.

If this is an issue for you, you can update the "insert_preferences_element" method to remove the br elements and modify the width of the main preferences element in "add_css_rules"

Otherwise, it's tight.