To repro:
1. Edit the content of an HTML Content module.
2. In the editor, select some text and then select, for example, "Title H3" from the Style drop-down. The text is diplayed as bold black in the editor.
3. Update and view the content outside the editor. The text is now non-bold and dark-red.
Problem is that the site's stylesheet is not used as the stylesheet by FCKEditor's editor area. I think the fix is to change Web/*Edit.aspx.cs to set fckInstance.Config["EditorAreaCSS"] to the URL for site stylesheet. Sorry I can't provide a detailed patch. It's not immediately clear to me how to determine the URL for the site stylesheet, and since I can't build mojoportal on Linux at the moment, I can't really test any proposed code changes.
BTW, it doesn't look like FCKEditor provides a way to associate multiple stylesheets to the editor area, so you might need to add the contents of fck_editorarea.css to the included stylesheets (eg mojo1.css).
--Dean