If you’ve recently updated to WordPress 3.8 you’ll see there is a whole new look, and included with that is a whole new colour scheme for the admin side.
Unfortunately I’m one of those folks with pretty bad eyes, and I don’t really like the white text on black background too much, it’s just straining on my eyes after a while. But if you read the screen that shows up after the update that explains everything that’s just happened, you’ll see that there are a bunch of choices for your colour scheme and you can change it right then and there.
But, if you decide later you want to change the colours again, where do you do it?
Simple – it’s now part of your user profile, just go there and make your choice.