I have a hot key set up to toggle light/dark mode. It uses the built-in macro, nothing complicated at all. On all but one computer this works fine, but it’s stopped working out of the blue on one. I know it’s registering the hot key, as I temporarily put in a notification. And yet, it’s not working.
So I don’t know if this is a KM issue, or a system setting in the OS.
Note that I have all the access toggle set to on for KM, no other issues with KM things. Any ideas what might be happening?
I have the exact same macro (mapped to hyperkey backslash) and it still works for me.
Your macro worked for me, using the same hot key, under MacOS 15.7.8 (Sequoia). If you state the OS version that your Mac is using, perhaps someone else will test it under that.
Well, it’s Tahoe .. but it’s odd because I have been running that for a while. It didn’t stop working with an OS update. It just … stopped working. But ONLY on that machine. All others also run Tahoe: working fine. 
Have you worked through this section of the Wiki?
Well, it could be that on that one computer you installed an app that grabs that shortcut. For example, Final Cut Pro.
Verify that the macro is triggering.
Do the other options (Set Dark/Light Mode) work.
Other than that, its hard to say. The action is a fairly simple AppleScript:
tell application "System Events" to tell appearance preferences to set dark mode to not dark mode