Add a class to preview containers

[TangentFoxy]

I'm using a CSS framework for a consistent look to my site, and SimpleMDE as a way to edit content for the site. In order to have the styling of the site apply within SimpleMDE's preview, I would like to be able to add a class to the divs containing the preview.

I've already figured out that they are classed editor-preview and editor-preview-side depending on which preview is being shown, but it seems these don't exist until a user clicks the button for them, so I haven't figured out a way to find and attach a class to them in JS.

(It looks like I can use customizing the toolbar to accomplish this, but is there a way to modify the default toolbar instead of replacing it?)

[sn3p]

duplicate #379

[margani]

see here.