Customize the Wordpress back-end

Hi all,

First time here! I have a question about customization of the Wordpress Back-end. I was simply wondering if any of you already had a bad experience or any advices. I saw some really great looking back-end on Theme Forest:


http://flatfull.com/themes/first/index.html
http://www.keenthemes.com/preview/index.php?theme=metronic_admin&page=index.html
http://demo.kopyov.com/crown/

And as pretty much everybody knows the Wordpress back-end I would like to make it look a bit better.

I saw on CSS-Tricks a way to add an external CSS file so it would overwrite the default style by not breaking anything. Does somebody knows about how to apply this CSS file to all sites of my network, may be by uploading this CSS file in all themes.

Thanks!