New TinyMCE CSS File

Create a new CSS file for the theme you're using and add it to the directory of your theme:

themes/example-theme/
or
sites/default/themes/example-theme/ (better)

You can call the new CSS file anything you like, in this tutorial we'll call it format.css.

Include the new TinyMCE CSS File

Log in to your site and go to:

admin/settings/tinymce

Add a profile or edit an existing one. Note: you need to edit all existing TinyMCE profiles.

Scroll down to the 'CSS' fieldset, and enter these values:

Editor CSS: define css
CSS path: sites/default/themes/example/format.css
* whatever the path to the CSS file you added in Step One
* path shouldn't start with a '/'

Submit the form.