In Office2013 theme the Margin property of ContentPresenter is bound to the Padding property of RadButton. So I suggest that you update Padding property of your button to unsure that the image is not chopped.
If you change the themes at runtime
, then you can add a style for the button in a Resource Dictionary which you can merge when you set Office2013 theme:
Afterthat you need to call it with DynamicResource in order the style to be changed at runtime.
I hope this information will help you and don't hesitate to write back if you have more questions.
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