The CSS-file that is used for displaying text in the HTML editor is set in WebStylesheet setting in web.config. The CuteEditor parameter EditorWysiwygModeCss is set to the WebStylesheet setting value.
<!-- WEB SITE STYLESHEET
CSS file for HTML content of Sitecore database.
The file pointed to by WebStylesheet setting is automatically included into Sitecore Client.
By using it, you can make the content of HTML fields look the same in Sitecore Content Editor,
and in the actual Web Site
-->
<setting name="WebStylesheet" value="/default.css" />