I guess by Touch and non Touch screen you mean the theme changing on click the "Switch to Touch" button in the UI for WPF demos application. If that so, the example is using our implicit styles theming mechanism which requires to merge the required resource dictionaries (the xaml files) with the controls styles. The number of the files that need to be merged depends on the controls used in the application.
So, the answer to your questions is, it depends. If you are using only controls from the Telerik.Windows.Controls assembly, yes you can change the theme using only a single xaml file. However, if you have controls from other dlls like Telerik.Windows.Controls.Navigation for example, you will need to merge them as well.
I prepared a small example demonstrating change in the Telerik controls theme. The used themes are Windows8 and Windows8Touch. Please give it a try and let me know if it helps.
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