Hi Tjardo,
1. Please try demo http://www.cutesoft.net/example/general.aspx, if it works for you, please download the latest version and try again.
Latest version downloads http://cutesoft.net/downloads/folders/21904/download.aspx
2. Second way is
Open file "\CuteSoft_Client\CuteEditor\Template.aspx "
Find section below and delete it
<link type="text/css" rel="stylesheet" href='<%=CuteEditor.EditorUtility.ProcessWebPath(Context,Page,"Load.ashx?type=style&file=SyntaxHighlighter.css")%>' />
Regards,
Ken