How to apply a theme to SQL Server Management Studio 2008?
I used this approach and theme provided here, it worked for me in SSMS 2008:

Beautify Your Management Studio 2005/2008


  To install, simply backup your [HKEY_CURRENT_USER\Software\Microsoft\Microsoft SQL Server\100\Tools\Shell\FontAndColors{A27B4E24-A735-4D1D-B8E7-9716E1E3D8E0}] registry key, and load in this new set (standard disclaimer about being careful with your registry, make sure you know what you’re doing, don’t blame me, etc). You can then either restart SSMS, or go to the Tools, Options, Environements, Fonts and Colors and simply click OK.


There is also a comment which provides the proper registry location for changing the theme in SSMS 2012, which only seems to differ by the version number.