By design, the main purpose of the StyleManager is to be able to set a theme to all our and most of Microsoft controls in your application easily. Therefore, once a Theme is set through StyleManager, it will be used for all user controls created after that.
For your case, I can suggest three approaches. In my opinion, the first one is the most appropriate and reasonable one. Here they are:
1. Use implicit styles and merge the necessary resource dictionaries from the desired theme, in the UserControl on which you want to apply a theme. Please, find the attached project which illustrates this approach.
2. As I wrote in my previous post, you can set a theme to the controls in the UserControl locally.
3. If you still want to use StyleManager, you should set an application theme in the constructor of all
UserControls. If you do not do that, the theme which is set to the last UserControl
will be applied to the UserControls which are initialized after it. This means that if you declare "UC1" with Windows8Theme, after that you declare "UC2" with VistaTheme and lastly you declare "UC3" without a theme, VistaTheme will be applied to "UC3".
Again, the first way to style your application is the most recommended one.
Hope this helps.
All the best,
the Telerik team