BuddyPress.org

There’s nothing about BuddyPress that makes it difficult to theme. The only reason most sites don’t deviate much from the default theme right now is simply because BuddyPress is still in it’s infancy and the people making sites with it are probably mostly hobbyists without a ton of design/css/html/php skills. There’s nothing stopping anyone from making a theme that looks nothing at all like the default theme.