With the new KitKat update, Android uses black color as System background. In JellyBean,it used to be a png image which resides in framework/drawable folder. But in KitKat, that png has been removed and now it uses solid black color. So here's a method to use custom image as your system background instead of black color.
2.Open drawable folder and find this xml file-
3.Now navigate to res\drawable-sw360dp-xhdpi (it may be xxhdpi for high-res devices)
4.Paste your png file here and rename it to tw_background_holo_dark.png
5.Now recompile framework-res.apk
6.Finally push META-INF and Android Manifest.xml in your compiled apk and then zip align it.
Here's a screenshot from my 4.4.2 TW KitKat ROM-