As first I want to say many thanks to Judy for her analysis and the hint of Fabio with the three files. I've the same issue on a fresh test installation of DNN 07.01.01. I'm preparing a new skin to upgrade our productive DNN6 portal.

The goal is, to use custom css styles from file at the editor provider DotNetNuke.RadEditorProvider.

After hours of reading forums, blogs and try and error it works!

DNN Version:            DotNetNuke 07.01.01
HTML Editor Manager:    DotNetNuke.RadEditorProvider

Variant a: CSS file at default skin path
HTML Editor Configuration:
CSS File:               /editor.css (the slash / is important, as Judy said)
Upload the CSS File to: /Portals/_default/Skins/_default (hint with the three files from Fabio)

Variant b: default skin at your skin path
HTML Editor Configuration:
CSS File:               ../YOUR_SKIN/editor.css
Upload the CSS File to: /Portals/_default/Skins/YOUR_SKIN

The hint behind 'CSS File' says, that the path is relative to its parent and base / is the root of your skin. I would say thats wrong. The base is /Portals/_default/Skins/_default.

It's that difficult, because there occurs no HTTP 404 error if the custom css file can't be found. It would be useful, if the hint behind 'CSS File' will be fixed or a button to check it.

Reto