@mehdi-w first of all, please be advised that the BP Default theme is going to be retired soon
per http://bpdevel.wordpress.com/2013/11/13/the-future-of-the-bp-default-theme/ and I suggest that you find another theme as BP is now compatible with nearly all WordPress themes right after activation. You might need to tweak some styles in some WP themes but in general, should be good to go.
Going back to your question, add that code at the very bottom of the default.css and
default-rtl.css files.