This plugin implements an admin theme which results in the Dashboard looking similar (but not exactly) to how it looked prior to the 2.5 release.
I got the idea to do this after looking at the Fluency Admin Theme, which doesn't support IE or I probably would have used it, and the Baltic Amber theme. Fluency redesigns the Dashboard and Baltic Amber offers multiple color choices for the Dashboard. After seeing how Baltic Amber loaded, I realized I could do something similar for the CSS declarations I wanted to change. It was surprisingly easy.