I have exactly the same issue. It's OE 3.2 and I try to make changes to extension/ezdhtml/design/standard/stylesheets/ezdhtml/editor.css. Absoloutely no effect whatsoever. I have also tried making changes to other stylesheets in case they were being picked up in preference to editor.css such as admin.css & core.css but the Editor style remains the same.
It's very frustrating because its not exactly the best WYSIWYG environment whereby the override stylesheet employed to render the final site bears no resemblance at all to what you see in the Editor!
After editing extension/ezdhtml/design/standard/stylesheets/ezdhtml/editor.css, go to IE/Tools/Internet Options/General/Delete Files/, check "Delete all offline content" and click ok.
If not, check that you can get this file from your server. Try to enter: http://your_site_url/extension/ezdhtml/design/standard/stylesheets/ezdhtml/editor.css
If your setup is ok, you will see the css text. If no check rewrite rules, there should be a rule like the following: