title: Separated .css file for Sitecore Rich Text Editor
body: <p>I'm trying to use custom styles in Sitecore Rich Text Editor. I have found article, which describes how to do it, but I have to place my styles in default.css file. 
  I would like to add my custom styles in separated .css file to avoid potential issues (for example, overwriting default .css file on upgrading version and so on).
  Is there way to use default.css and custom.css files at once?</p>