Unlike Windows Phone or iOS, Android looks quite differently on various devices. OEM skins such as HTC Sense, Samsung TouchWiz, or the lesser known Emotion UI look totally different than barebones AOSP. Clean Android might have lots of fans because of its simplicity, but not everyone is a fan of the default UI.

Almost every OS flaw be fixed either by the source amendments or by using Xposed Framework by XDA Senior Recognized Developer rovo89 and his partner in crime Recognized Contributor Tungstwenty. If you want a nice looking expanded notification background, XDA Recognized Developer serajr prepared something that might be of interest.

Blurred System UI is an Xposed module that blurs the background of the Expanded Notification Panel. After applying it on your current ROM, you will see a partially transparent background, which looks quite a bit better than the standard solid background used in many ROMs. This module lets you to configure a few things like bitmap scale and blur radius, so you can get the expected results relatively easily.

The module currently works only on smartphones, but tablet compatibility might be added in the near future. It's reportedly working with multiple ROMs---both skinned and AOSP-derived. You can find the module and Gaussian blur your notification panel by visiting the Blurred System UI module thread.