Here is what to do:
1. Add the following elements to the respective ConfigFile.xml or ListConfigFile.xml, which are located in the /Program Files/Common Files/Microsoft Shared/web server extensions/wpresources/RadEditorSharePoint/5.x.x.0__1f131a624888eeed/Resources/ folder:
function OnClientLoad(editor, args)
//here you can modify the text area style
editor.get_textArea().style.font = "13px Verdana";
I hope this helps.
the Telerik team
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