From Pearsonified:

What you need, my friend, is a futureproof, bulletproof means of protecting your CSS mods so that you can ride the wave of agile development without the associated headache from upgrading!

Never touch the original stylesheet again — it’s just not the most secure, most efficient method of operation! On top of that, it would be nearly impossible for you to go through a stylesheet that you’ve hacked up and pinpoint each and every little thing you’ve changed.

How to Protect CSS Mods for ANY WordPress Theme>>