After adding a nice white-on-black theme to Eclipse for my PHP work, I decided I wanted a similar theme in Visual Studio 2010 for my C# .NET development. It was pretty easy to do.
First you need a new theme. Go to the studio styles website (studiostyl.es), which has over 1000 styles for Visual Studio. Choose one or more and download them to a folder you can find. The c:\documents and settings\yourname\my documents\visual studio 2010\settings is a good place for them.
In Visual Studio 2010 click on the Tools menu, Import and Export Settings..
Import Selected Environmental Settings..
Make a backup for safety..
Select your theme or Browse for it..
There you go!