@gnarf37 was complaining that the theme needs to be set on each child of a controlgroup individually instead of it being possible to set the theme on the controlgroup itself, and have each child inherit the theme from the controlgroup.
To be honest, having any arbitrary parent accept a data-theme or data-content-theme would be useful too.
@gabrielschulhof - Shall we try to add this for 1.3 together with changing controlgroup into a widget?
Changed the milestone to 1.4 because it's not easy to implement. We are planning on looking theme inheritance for 1.4 so this is not something we will land in 1.3.
+1 Would be super useful!
Controlgroup: Added option theme. Fixes #5301.