I am not new to OpenCart, but I never try OpenCart after version 1.5.6. I can see there are a lot of changes during this time. When I wanted to add a new theme, I copied my theme into the directory catalog\view\theme, but there is no link to select the theme into System >> Settings >> Edit >> Theme. Even it is not listed in Design >> Theme Editor >> Choose your store.
Although I am going to dig into deeper about this new release, during this time any help will be appreciable. And importantly there are few google searches which all are not about the specified version.
N.B. I know OpenCart added .twig template in this release and my new theme is compatible with OpenCart v-3.0.2.0