My mistake...transparent backgrounds are unsupported...still needs a hide shellimage option or/and translucency support on shell images.
This could be a neat and handy feature if used right.You could have a dark window skin so that light Text-Normal would be on dark areas of the skin...but the window could be light with dark text if there was a font color choice for explorer backgrounds.(there used to be a feature like that at one time)
And of course,the opposite scenario could have uses too...light to dark,dark to light.
I like background options...always thought they were a good idea,but as with all WB features...they need to be implemented correctly or they are just another way to screw up your blind.