This change seems to have caused a regression for the -[NUIButtonRendererBackgroundColorTests testBackgroundColor] test. If you fix the regression I'll be happy to take a look.

I have already updated the regression test for both UIButton and UITextField. Additionally, I added support for UIActivityIndicatorView with its own test as well. Let me know if I need to update something else.

Looks good to me, but just a word of advice for next time: Please try to keep pull requests as small and modular as possible. I noticed 13 hours ago changes for UIActivityIndicatorView also snuck in. In order to avoid that, please place changes for other components in a new branch (as opposed to merely adding onto your master branch) and submit a new pull request for that branch. You can reference pull requests made by alexeyt820 as an example of how it should be done. The reason we want to do it this way is because it reduces the risk associated with merging in a pull request when it's smaller. Also if you happened to introduce something that doesn't get past review with the UIActivityIndicatorView changes, the entire PR would be placed on hold rather than just the relevant part.