Actually the CssFiles property works only for Iframe mode, because the div mode is part of the current page and all styles are directly applied to the contents of the div.
My suggestion is to load the EditorContentArea.PD_RadSkin_1.css file through the ContentAreaCssFile
property as shown in this demo: Setting Content Area Defaults
. The name of the file should not be a problem, but just for the test rename it to a word without punctuation symbols as RadEditorSkin.css.
If you still experience the problem isolate it in a sample project and send it for examination by opening a support ticket.
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