RadMessageBox has title and message ui elements inside it which have specific fonts sizes set. If you need to modify the fonts or the font sizes you can create a new control template for the RadMessageBoxControl class.
In order to avoid writing it from scratch you can copy the default RadMessageBoxControl style and modify its control template.
You can do this by extracting the style from the Telerik.Windows.Controls.Primitives.dll assembly with the reflector tool. The themes are located in a Resources folder inside the assembly.
You can also use the control template from this help article.
The special namespace looks like this:
Write again if have other questions.
the Telerik team
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