Firstly, to style the RadRichTextBox dialogs, you should use implicit
styles in your application. This will say that you have to use NoXAML
binaries and external themes
in your project. How to do that, you can read here
Secondly, the RadRichTextBox dialogs are composed by other controls - RadButtons, RadListBox and other Telerik RadControls. The dialogs also contain default Microsoft controls such as RadioButton, CheckBox, ScrollViewer and etc. These default Microsoft controls are styled in the System.Windows.xaml
In order to create your own custom styles, you should style the controls they use first. You should create these styles based on the previously created ones with the same TargetType
This will say that if you want to set the Background of the RadButton control to Blue and keep all its other properties, you should do it like this:
You can read more about how to base your own styles here
In addition, you can see the styles' keys in the respective xaml
file which contains the control's style.
For your convenience, I have created a sample demo project which illustrates the above described approach. It is attached in my post, so you can see it.
Hope this helps. If you have any other questions, please get back to us.
the Telerik team