The RadEditor's dialogs are based on RadWindow which loads its contents inside an IFRAME. This is another document which does not inherit the css stylesheet of the parent page where the editor resides. To load the needed css classes in the dialogs you should set the DialogsCssFile to point to external css file which contains the stylesheet of the controls contained in the dialogs.
You can find information about setting the DialogsCssFile property and the needed classes that you should load in the following article: Registering an external skin of RadEditor.
All the best,
the Telerik team