How does one do that? Through the Themes icon in the menu bar, under Preferences, I was able to double click on the following themes. Adapta-Nokto, CBlack, and Windows 10 Light Theme.

There is a checkmark to the left of them indicating they are installed. But I can't seem to activate them. I double click on them and nothing happens. Right mouse button and middle mouse button does nothing.

When I go back to the Themes tab under Simplified Settings, they do not appear in the Style drop down list.

How do I activate these themes I downloaded? I see they are in ~/.themes as expected.

~$ ll ~/.themes
drwxrwxr-x  5 clu clu 4096 Sep 11 19:49 ./
drwxr-x--- 26 clu clu 4096 Sep 11 19:55 ../
drwxrwxr-x  9 clu clu 4096 Sep 11 19:48 Adapta-Nokto/
drwxrwxr-x  7 clu clu 4096 Sep 11 19:48 CBlack/
drwxrwxr-x  9 clu clu 4096 Sep 11 19:49 Windows-10/

So I found an option under Themes Tab called Desktop. When I click it, the downloaded themes are available to select, along with a million other ones. I selected each of my themes but really didn't notice much difference. The Start Menu changed a little bit. Very underwhelmed. Is theming only available for the Start Menu? No title bars, windows, background, icons, mouse looks different at all.

I must be doing this wrong. ??? The Mouse Pointer, Applications, and Icons buttons don't list the new theme as an option to select. ?? I guess I mistakenly thought a "theme" impacted most aspects of the GUI. ??