Go to settings, search for Application Style, there, in the top right you can click Gtk Theme and select one. You can also click Get new.. and download a new one. Note that they might now show all by default.

You can find more via the website. To do so, select any, click Visit project website and search the site for more. To find them in the Get new... window, just put the name in the search bar. They are guaranteed to show then.

Edit: Any of the items I named might be slightly off. Need to get to my computer to correct that 😄