I found this blog -- customizing rich text editor, but what I don't understand is the part where the author writes
Set the PrefixStyleSheet-Property of the RichHtmlField. The classname has to be lowercase!
<PublishingWebControls:RichHtmlField id="Content" FieldName="PublishingPageContent" runat="server" PrefixStyleSheet="my-style-must-be-lowercase"/>
Where would I insert the above code? In the masterpage? Pagelayout?