How do discourse support themes?



How do discourse support themes? Or, how could I modify this appearance?
I’m asking this because I took a glance on all the Discourse powered sites and find out that those are all using the default theme and, well, maybe those sites have it’s own modification on the inner code, but that makes no difference on the outside look of them.
And go through the Discourse Meta, seems like nobody is developing a theme…

And by the way, to become a nice Open Source software, a official store for all the plugins might be necessary?

Discourse is truly powerful,
looking forward to use Discourse to power our community :smile: