This is an issue that I have come across many times and I really liked this last solution I found from this site.
I use the JCE pro and had a little trouble following the article, but the author did a great job putting the article together. Here is where the “Styles” area is in the Pro version.
Components >> JCE >> Profiles.
In the Profiles section click on “Default“. Then go to the “Plugin Parameters” tab, and scroll down to the “Styles” section. Click on the “Styles” tab and the right side of the screen will bring up settings.
I was localhost and I just added the new CSS to my “custom” style sheet. I went back to the article and set the style of the IFrame I had already inserted through the JCE Pro and added the new style to it.
I hope this helps someone else out there…