It’s called “Plasma theme”, and it is set in a different section of the Appearance page in System Settings. It is independent from the global theme, although changing the global theme will also change the Plasma theme.
If you do not find a Plasma theme to your liking in System Settings, then you can look for one at store.kde.org, but be sure to pick one that’s still being maintained. Plasma themes from 10 years ago are bound to break your system.
Also, an easy way to install it is to first install ocs-url from the AUR, and then you can install the theme directly from within your browser by clicking the OCS link on the theme’s page.