So you can customize any theme without loosing ability to update theme without losing customization.
No, why this post? There is quite enough resources on web for this issue!
That’s because I’m using this instruction quite often during some site customization or developing a new theme. All the default code is not memorized yet, so I have to use some good old fashion copy/paste technique. And I have this blog in Bookmarks on every browser I use, so there is no need to disturb the almighty google for such insignificant issue.