Changing background on apps/widgets screens

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http://forum.xda-developers.com – Let's say I have a theme I really love except that the background for apps/widgets is plain black and I want background to be the same as the wallpaper. Given that there is no setting for this, I assume I will need to learn how to open theme, replace plain background with wallpaper and reassemble theme. Yes? What are the likely names of the home screen and app/widget backgrounds and where in res would they be? They're going to be a png file, yes? I assume with this information and Photoshop I can follow instructions in how to theme thread to actually accomplish this task. Is there any (HowTos)