Friends,
Let's talk about
Plugins Pre Format 
If I understand correctly, you want to save our own pre-installed styles for each plugin, right???
If yes, we can propose next:
To each plugin add some button with name "Save Preset Style" and when you or your client adding a new plugin from the list (Add new Plugin Modal) plugin comes with saved previously preset styles
I think it's possible

Users will be available to create own UI styles
I want to note, that Preset Style will include only CSS styles: colors, backgrounds, borders, etc....
Regards,
Artem, Balbooa.com