Use jquery.sceditor.mybb.css from plugin sceditor for Mybb.

jquery.sceditor.mybb.css from https://github.com/samclarke/SCEditor-MyBB/tree/master/jscripts/sceditor

Better use jquery.sceditor.mybb.css instead jquery.sceditor.default.min.css has better style for blockquote, blockcode etc..

jquery.sceditor.mybb.css
http://i.imgur.com/V7u7TiE.png

jquery.sceditor.default.min.css
http://i.imgur.com/hZUOtKZ.png

And i don´t recommend use minify file in this case, because this file is used too to put style for pre tag (used in spoiler tag) etc. Simply put this file is often used for customization.