As this thread is rather old, the ways to change the style settings in RadRichTextBox have changes.
In case you wish to set particular style settings to the editor so that all documents shown in it will use those settings, you can refer to this help article
I suggest you also get familiar with the styling concept of the control as it provides a way to easily apply different sets of styles to parts of the document. You can check this article
to learn about the types of styles, their creation and evaluation and here
you can read how to use the UI of the feature.
I hope this helps!
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