When I use a standard Windows aero theme in Windows 8, the VS2012 lets me to choose between light and dark themes, but I created myself a convenient high contrast theme in W
As you've found, the High Contrast theme in Visual Studio is automatically linked to enabling Windows' "High contrast" mode in "Ease of Access".
While you can't change themes while that mode's active, you can still change some color settings (editor, windows, etc.) directly via Tools -> Options -> Environment -> Fonts & Colors. Many settings come directly from Windows, but you still have some options here.
Not for this scenario, but just FYI for anyone intrested in tweaking themes, the Visual Studio 2012 Color Theme Editor extension is pretty handy. Scott H. has a post on using it.