There are a few on addons.opeera.com ... they were called "skins" and searching for that term brings up the few they have (and a few themes that just happen to have the word "skin" in the title, of course).
https://addons.opera.com/en/search/?query=skin&type=themes
Otherwise ... if you're looking for any in particular, just ask. I still have some in my Opera 12 profile; I'm sure others do as well.