The RadEditor content area is separate from the rest of the page and its CSS is also separate. As you already found out, you can use the <cssFiles> collection in the editor tools file to select which stylesheets will be loaded in the content area. By default these are HtmlEditorTableFormats.css, HtmlEditorCustomStyles.css
, and CssEditor.css from the editor Resources folder. If you set <cssFiles>, then only the files you list will be added to the editor content area. If you want to have the default stylesheets and your custom one, then simply add them all as <item> entries in the <cssFiles> element.
As my colleague suggested, you can use Firebug for Firefox or IE Developer toolbar for Internet Explorer to inspect the editor content and see if any custom styles are applied to the paragraphs and if there are, add them in your custom CSS file.
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