BuddyPress.org

WordPress allows you to subscribe to the wp_head filter to add things through plugins to your site. More information is found here on that. An example of this in use would be looking at this one even though it is a little different than you are looking for, but gives idea how to do it.

Really, you create a little plugin to put in your /mu-plugins/ folder that has a function that adds the stylesheet to the wordpress header of every page. Then you subscribe that function to the wp_head filter.

Trent