How to find Frontend theme Applied to masterpage

Hello Team,
Thanks a lot for your Support !
I need your support again, we are facing a problem that how we can find the Frontend theme Applied to the Master Page (Page template). Here we have two Themes for the master page: Full Screen Theme and Center Aligned theme. i want to find which theme was applied to the master page(page template) programmetically, to change some setting.
please post a sample code as asap.

Thanks and Regards
Ankit Rastogi