Ever found a really good theme that was just short of greatness? All you had to do was fiddle with a bit of css here or there and it was just perfect.
The only problem was that when a newer version of the theme was released, all those changes would be lost. You would have to carry them out again, remember what you had changed and how. If you were technically savvy, you maybe even created a patch which may or may not apply any longer. Even without updates, maybe you wanted to cancel one of your customisations, but it was not always easy to remember what was changed where.
Such small customisations were the bane of any theme user, however help is at hand with local.css, which is being used for more and more themes to keep local customisations separate from the rest of the theme.
With Arthemia for Drupal (and some other drupal themes too), all you need to do is place your customisations into a separate local.css file and drop it into the theme folder. Come update time, that file should remain untouched, leaving your theming customisations in one place.