Can't fix/remove white border in classic context menus in dark mode.

I have been having this problem with just one of my computers. I think it started a year or so ago when I upgraded to Windows 11. The problem on this machine is I have an ugly white border around the right click context menus. (I am using the classic menus - the new styling was fixed according to a tutorial here.) I keep thinking I will find an answer yet nothing helps.

What I've tried:
Accesability settings - they are fine.
Changing themes - no difference.
Repair install of Windows 11 25H2 - no difference.

And lots of other things. This is what I am looking at. My laptop on the same build has no white line around the context meus...

Windows 11 Build 26.200.6884 - 25H2

Hopefully someone can give me a hand with this. Incidentally I am not using any tweakers or Customizers, just bare Windows 11.

Thanks in advance.