How to apply a theme to SQL Server Management Studio 2008? Has anyone had any success with, for example, applying a 'dark' them to the Text Editor component of of SSMS 2008? 

There was an old trick for 2005 which is no longer operational and has not been upgraded.  http://winterdom.com/2007/10/colorschemesinsql2005managementstudio 

My eyes will thank you very much!  Cheers.
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.