Here is a Xposed Module I have been working on. It is in alpha stage with a few features for now but will definitely get more as I learn more. I call it XBackground and of course requires the Xposed Framework already installed. I am sharing it now even if it is not fully developed to get feedback and suggestions.
XBackground - is an xposed module that runs on top of the xposed framework and its main function (for now) is the ability to let you change the Holo Dark background and Holo Light background used by some stock apps as well as many apps from the Play Store to any custom image from your Gallery or any color you like. It also can replace your Notification Panel background to any color/transparency or to any custom image from your Gallery.
1. Download and install, then activate it in Xposed Installer and reboot.
2. To use, just run the app and say you want to change the holo dark background, just tick the color and choose the color you want from the color picker, then soft reboot. Or if you want to use a custom image, untick the color option and choose the image from the Gallery then soft reboot.
NOTE: Changing from color to image and vice versa always require a soft reboot. Changing from one color to another color always require a reboot. And changing from one image to another image does not require a reboot.
Credits: @rovo89 for the amazing Xposed Framework. BIG THANKS.