It is possible to change the background color either in SkinStudio or in the Windowblinds, Modify style area. However once you do it will effect the background color of other programs also. To easily test what it does to your system/programs, go to the Modify style" area of Windowblinds, click "Change system colors" and then scroll down and click on "System Colour: Window". Change the color to what you want, click Close Window and then click "Apply style to desktop". Then check out all your programs to see what it's done to them. If you don't like what it has done, just put that color back to white (which is preferred by most programs). Good luck.
To add to LightStar's reply. I know for sure it affects these programs and areas: Notepad, Firefox Library "Box", Object Desktop and boxes, such as Properties when you checking for a programs general, shortcut, details etc. But also drop down boxes and also these kind of boxes (or at least in WindowsBlinds).
I also noticed right now it changes the colour in the Save As field when I saved this below picture to easier show what kind of boxes I was referring to above.
So if you planning to darken your Notepad, make it a bit lighter then I did above, had to lean closer to screen to see what to save the picture as !