It depends on context I think, with my main tank, no background black sheet and you can see a freezer and ironing board! Chap here had a beautiful dining room with walls painted a rich natural soft green, that looked fantastic no background sheet necessary. Show us a before and after and we might know better, reflection can also be an issue depending on the choice of background.
Please don't be offended, but longer term I think your tank is too small for an Angel fish, the Cardinal tetras are an excellent choice. Your plants look very healthy and attractive, you might consider some mid water fish with colouration to complement the plants and contrast with a background paper - I like classics black/green red/green.
Good luck.