How do I apply a custom style to the FontPropertiesDialog (to start, but all of the exposed dialogs in the future) used in the RadRichTextBoxRibbonUI? I want to make the dialogs appear as if they "belong" to our application...
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 file.
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: