This issue is just a backport of MDL-37674:

Depending on how you've styled your blocks, a block that has no header can lead to a missing top border on block content. It would be really nice if there were a class on the .block element that indicated that there was no header, so that if, for example, content then needs to be given a border-top and a border-radius on the two top corners, theme designers have a style with which to target those blocks.
The .block_course_summary block has no header by default. Another block which can have no header, if a header title is not entered for it, is the HTML block.
I am able to add a .no-header class to the .block_course_summary block with the patches above below, but am not able to get it working for the HTML block.

git checkout MOODLE_25_STABLE
git pull git://github.com/samhemelryk/moodle.git wip-MDL-37674-m25

git checkout MOODLE_24_STABLE
git pull git://github.com/samhemelryk/moodle.git wip-MDL-37674-m24