I use Vundle myself for handling my plugins so I wasn't really
questioning plugin managers as such. It's just that all the themes
I've used were a single file dropped in the colors directory and they
haven't had dependencies complex enough to really require the plugin
manager. Taking a second look at evergreen, it seems the theme
integrates with a few other plugins and that's why the manager is
recommended so I can understand why a that is.
Since I haven't ever felt the need for a fancy status bar or other
graphical features besides syntax highlighting in colours I like, this
has never been an issue for me.