Dashboard CSS Tweaks

I'd like to make some CSS tweaks to the Dashboard, for example darkening the top gray bar. (If I'm not mistaken, Edublogs does something similar, as their Dashboard looks very refined.)

What's the best way to do this without risking any modifications being overwritten during an update?

Also, if I include a custom page in the Dashboard by creating a new page (for example, if I create a custom help/tutorials page), will it remain in an update?