Steps to reproduce the issue

Take any extension which uses the Bootstrap Thumbnails feature (http://getbootstrap.com/2.3.2/components.html#thumbnails) and see how ugly that looks in staging.

As an example you can use my component SermonSpeaker (eg http://www.sermonspeaker.net/download/sermonspeaker-component/sermonspeaker-component-5-5-4.html) and in backend go to "Extensions -> SermonSpeaker). That brings you to a "dashboard" view using Thumbnails.

Expected result

Actual result

System information (as much as possible)

Joomla 3.6.5 vs staging

Additional comments

I think this comes from #12643.
That PR added a block to restyle thumbnails here: https://github.com/joomla/joomla-cms/blob/staging/administrator/templates/isis/less/template.less#L1523-L1692

I think those rules need a id or class prefix so they only apply to media manager related output and not all thumbnails in any extension.