The code includes the DNN 4 css is as following:

 

    Dim s1, s2, s3, s4 As String
    s1 = Common.Globals.HostPath & "default.css" 'default css
    s2 = PortalSettings.ActiveTab.SkinPath & "skin.css" 'skin css
    s3 = (PortalSettings.ActiveTab.SkinSrc).Substring(0, PortalSettings.ActiveTab.SkinSrc.LastIndexOf("/")) & "/skin.css"
    s4 = PortalSettings.HomeDirectory & "portal.css" 'portal stylesheet
    cntlCE.EditorWysiwygModeCss = s1 & "," & s2 & "," & s3 & ", " & s4

 

If you want to add your own styles, [lease add the EditorWysiwygModeCss="/somefolder/some.css" into the CuteEditor section of DNN web.config.