You don't need to be a partner to make a custom channel background, but you do need to be a partner to have a banner, which is a customisable area above the video player.
Making a custom background is easy enough, just download Google's free Picasa photo editing software. It has a 'create collage' function which is not very straight forward but, with A LOT of patience - and trial and error, you can create a decent enough channel background for free.