To prove that theory, I downloaded oreo-grey-cursors.tar.gz from this page (the highest rated cursor theme from that site), extracted the contents into my /usr/share/icons/, saw Oreo Gray Cursors appear as a choice, chose it, and saw it go into effect.
So it worked. Thanks!
Only, this new theme also exhibits the “resize icon does not change to text insertion icon over text editors” problem from my other post.
So the only normal-working theme for me at present may be Adwaita.
What would be the crucial attribute of Adwaita (which makes it work normally)? In other words, how could I find other themes like Adwaita in this respect. @soundofthunder may have said Adwaita is from GTK.
This theme, or its “Simp1e-Adw-Dark.tar.xz” variant, was problem-free (maybe “Adw” is why): Simp1e Cursors - Eyecandy for your XFCE-Desktop - xfce-look.org
I might add that, other than this Oreo, I did not download any of the other themes, i.e. they came with the OS install. I wonder at incompatible or abandoned themes creeping in that way.
And here’s something very odd. After testing of Oreo, I see that Simple-and-Soft is given some effect (e.g. the main pointer icon is that of Simple-and-Soft). So I thought perhaps I was being inattentive and did not notice that before. To confirm, I went to a VM in which no installing of a new theme took place, and there choosing Simple-and-Soft does not give me its main pointer. This whole area seems rather random.