Custom background color

It appears that all themes are either based on the dark or light theme. Thus all the backgrounds (text editor, solution explorer, output window, ...) are either black or white. How is it possible to create a custom theme with a different background color? It seems not even possible using the new Visual Studio Color Theme Designer.

[PXLcat]

Solarized (Light)'s background is FDF6E3. Too much corresponding entries in the "Show all elements" view though, could'nt find the corresponding one.