iOS 7 themes - why is mine white? I thought with iOS 7 the theme was supposed to be based on the colour of the device. If this is the case why does my black iPad have the white theme.

If this is not the case how do I change it to a black theme?
Only the iPhone 5C has any theme support, in that they contain custom wallpaper to match the colour of the device which is selected as a default when first using the device.  This themeing does not extend into apps, and is simply a pre-chosen wallpaper.  It has no relevance for any other phone model, or iPad.