When version 3.8 of WordPress came out, it changed a number of things in the admin area. One of those things was the default color scheme.
It used to be that the default color scheme was lighter in color.
Now it’s much darker and looks like this:
Changing the Color for Multisite Users
While it’s easy to go to your profile and change that if you don’t like it, if you run a Multisite Network, then you have all your users to deal with as well.