AppleChronicles

I am not a fan of tweaking an operating system. This can cause no end of problems if you are not extremely sure of what you are doing. And even if you are sure, the next OS update can be a problem, or cause a problem with the tweak.

Having said that, Tinker Tool will allow you to make many customizations in a relatively safe manner.

I did make one manual customization to my Mac. I gave it a custom startup screen. If you wish to do this, you replace the jpeg file at this location:
/System/Library/CoreServices/DefaultDesktop.jpg