This issue has been fixed back in Q1 2012: http://feedback.telerik.com/Project/154/Feedback/Details/111022
. I have tested both approaches - setting the Font of the control and setting the Font of the ButtonElement, and both are working properly on my end.
Should you experience any issues setting the font of the button, please get back to me with information how to reproduce the issue and I will look into it.
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