I have had the same issue. Use should keep your Marketo system css. After that code import a css stylesheet file. add most stuff here and not in the code of the template itselve. This should make it work in edit mode, preview mode and live version. However I still have some issues with TinyMCE displaying regular H1 style, rather than my custom H1 style. This issue is for landing pages only. An email template has inline css and therefor displays perfectly in TinyMCE.