Changing the css of admin panel?

Hi,

I want to change the css of the admin panle and the back-end. Where is this exactly? Is it in the theme or somewhere else. In a multisite install is there a way to change the css so all subsites that will be created have this specific css regardless of whatever theme they use or shall the change be made in each theme for consistency. Also when there is an update to WordPress will this affect the css change?

Many thanks