I have customised Bootstrap theme to appear as responsive layout with fixed max width. It means that on the large (24" screens) page has fixed width but it changes layout when it's viewed it on mobile devices. It's all working fine until "turn editing on" is enabled. The main issue is mce editor - please have a look on attached image to see what I mean. for some reason I'm not able to control width of this element. I have tried your option with adding custom css to config.php but it didn't work.
Here's the front end site link (responsive) http://yourdecision.oten.tafensw.edu.au/
And attached image shows how mceditor looks with "editing" is turned on. Would you be able to help me?