In the following help article you can find information on modifying the CSS Styles Drop Down http://www.telerik.com/help/aspnet-ajax/css_styles.html
. Keep in mind that since you are in WSS 3.0, you need to modify the respective ToolsFile.xml of ListToolsFile.xml (forms scenarios - list, wiki, blog ...), which are located in the /Program Files/Common Files/Microsoft Shared/web server extensions/wpresources/RadEditorSharePoint/5.x.x.0__1f131a624888eeed/Resources/
I hope this helps.
the Telerik team
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