First of all: Thank you for doing a great job here!

I think the behaviour and the styling of TabBar and TabButtons in Universal style are a little bit strange, especially in comparison to the standard and the material style.

1. The tab buttons are reordered every time when selecting a tab, with the selected tab on the first position. I never saw such a behaviour before. If it is intended, I would suggest to implement an option for being able to turn it off.
2. You use a quite big font size for the TabButtons, especially in comparison with the font size used in other buttons / controls.
3. In Material and standard style, the currently selected tab button is highlighted using colors and/or a line below the button's text. I personally would prefer a similiar highlighting style for the Universal style too.