I would say that you could change that in the body.mceContentBody class in the editor.css. Normally the CMS should take the editor.css located in the template folder but for me it takes the file located in the cms/css folder for some reason.
According to http://www.silverstripe.org/customising-the-cms/show/276090?start=0#post280958 there is a bug in the current silverstripe versions which prevents Silverstripe to load the editor.css out of the template/css folder. A fix for that bug is given there, you might want to follow this. (It's always unwise to change the core code since it will be overwritten once you upgrade Silverstripe)