Google octopress themes, I found two websites list many beautiful themes.

I choose a theme named DarkStripes.

The installation is very easy, use this theme as a example(from its README):

$ cd octopress
$ git clone git:// .themes/darkstripes
$ rake install['darkstripes']
$ rake generate

NOTE : When I installed this theme, it overrited my about.html in source/_includes/custom/asides/about.html .

The offical docs also provide a 3rd party themes list: