[O] How to make this horizontal line in org-super-agenda

[Sven Bretfeld]

Hi

Does anybody know how alphapapa made these horizontal lines with
separate the agenda blocks? It can be seen in the "after" screenshot of
the github main page for org-super-agenda:
https://github.com/alphapapa/org-super-agenda/blob/master/screenshot-after.png

I cannot figure out how to get those lines. Are they
color-theme-related? Is it a function which continuously escapes me?
Apparently it's not connected to
org-super-agenda-fontify-whole-header-line.

Thanks for help,

Sven