When setting the DialogsCssFile property, the generated markup of the RadEditor wrapper changes from 

Radeditor RadEditor_<MyCustomSkin> reWrapper

to

Radeditor <MyCustomSkin> reWrapper.



Since the custom skins generated through the Theme Builder requires the Radeditor RadEditor_<MyCustomSkin> reWrapper class syntax, they won't be applied to the RadEditor body.

The string <MyCustomSkin> is the name of the custom skin.


To fix the problem explicitly set the RenderMode property of RadEditor to "Lightweight", i.e.

<telerik:RadEditor RenderMode="Lightweight" DialogsCssFile="~/MyCustomSkin/DialogContents.css" ID="RadEditor1" runat="server" Skin="MyCustomSkin" EnableEmbeddedSkins="false">
            <ImageManager ViewPaths="~/" UploadPaths="~/" />
</telerik:RadEditor>