GTKs Themes Location: [ /usr/bin/themes | ~/.themes | ~/.local/share/themes ]
You choose Pakit Theme in GitHub: https://github.com/refi64/pakitheme
Run pakitheme install-system and pakitheme install-user to grab your current GTK+ theme and install it as a Flatpak into either the system or user installation, respectively. You can also pass a theme name, e.g. pakitheme install-user Adapta, and pakitheme will install that theme instead of your current one.
Use pakitheme clear-cache to clear the theme storage cache.
