Change television screen look when it's set to off

Hi,

since several days I'm trying to find a solution to change the look of the television screen (not the frame!) when it's not on. E.g. another color than black or another shader (e.g. glas) or a texture. But I can't find an option to do this. I already tried to change the shader from "videosurface" to several other shaders and tried to edit the alpha of the items of the material of the screen.

Can someone please help me? :/