Yes, Adwaita is a default theme, available to any user, and it is found in /usr/share/themes/
If you want to edit the colors in it, as I described before, copy it into /home/user/.themes. You should rename it AdwaitaEdit, (or whatever name you choose) so that you know which one it is on the list in Controls. Then after you edit the colors, change to AdwaitaEdit in Themes > Controls and you will have your new colors.
Of course, you can do that with any of the themes listed in Themes > Controls - not just Adwaita. You can download gtk themes until you find one that is closer to what you want, so you have less editing to do.
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