The new and improved now playing screen includes a non-gui option to change the colours to a user defined option (the default is to follow the gtk theme)
to change the colours:
open for editing the gmpc.cfg file located at the ~/.gmpc folder
locate the section titled [Now Playing]. There you can change the colours of both the background and foreground colours using standard colour names.
for example for a black background change the default values to background-color="#000" etc...
In addition make sure that the use-theme-color is set to "0" (use-theme-color="0")
here is a black colour example: