This is because the "-darkback" module style is only available in the emusica template. You'd have to use this template or just use the "-dark" module style in this template Incline that you are using.
thanks. do you mean the "dark style" theme style? there is also no dark background. it looks just like this:https://www.dropbox.com/s/hf5l6rsp05l59p5/mp3gallery-dark.jpg?dl=0
when typing "-dark" in module class suffix nothing happens...