You need to activate the snippet in order for it to display. First copy the checkbox.css file to your vaults /.obsidian/snippets folder, then in Settings - Appearance, scroll to the bottom and toggle checkbox on.
I did activate the snippet in my normal vault and a test vault to no avail. I swapped between a few themes (Things, Default, etc.) Even did a couple of restarts to make sure the problem was not between the keyboard and the chair.
I did a bit more research and did find another snippet from the Obsidian ITS Theme called S-Checkboxes. That worked straight out the gate.
But I like the subtle appearance of death-au’s, but I am unsure of what to tweak to get it to appear.
That’s odd? They both work fine for me. Perhaps it was not properly formatted. Try copying it from Github again but copy from the raw view instead. (click the raw button first)
Never mind, I just looked at this a little closer and that snippet does not work properly for me either. The default checkmarks are visible on top of the changed items so there is something wrong with it.