Im using smartdark for my theme for my site, and now I want to use the sublack theme for just my forum only.

I downloaded the phpbb sublack theme and went to my admin cpanel then to forums. From there I went to install theme sublack, BUT it still only shows my smart dark theme

as far as I know I uploaded the theme to the forum/template dir and it should work

I know i did or did not do something right, so how do I properly install a forum theme?