The drag and drop to which you refer is not actually related to the theme but in fact the course format code (configured by '/course/format/topics/format.js' etc.) because you are moving course sections and not blocks. As a test (no sound) I tried something similar in M2.5:
As you can see it can be a little temperamental, but does work in Chrome. I seem to remember some discussion about a bit of the YUI code being broken but cannot remember where on the forums / tracker this is. The issue is a YUI one and not a Moodle one.
When you drag a section inside a section because you cannot have sections in sections (unless you use Marina' Glancy's flexible sections course format - https://moodle.org/plugins/view.php?plugin=format_flexsections) then it will put in a link.
As things improve, perhaps export the course and try on a development M2.5 server and see if you get the same results.