Overwrite only CSS of default theme

Is it possible to overwrite only the CSS of the default theme? I would like to style a few elements in the admin using CSS, but if I copy the entire default theme it's an overkill (just to change a few elements with CSS I would copy all the template files?!) but if I just edit the default, I think I won't be able to upgrade.

So what's the best way to do that?

This is hard to decipher. Yes, you will be able to upgrade. You just have to be careful how you upgrade, that's all. Perhaps someone else will understand exactly what you want.... lol.