Unfortunately, you can't put in a background for a map that doesn't have one already, as that would require the .osu files to be altered, and then it wouldn't count to your online rank (unless this doesn't matter to you).
What you could do is put a playfield.png (a greyscale image that is effected by the set background colour and is used for the background when there is none) into the map's folder.
Or just use/make a skin with a playfield you don't mind looking at. I get the feeling you're using the default skin, since you apparently find the playfield ugly. If you like the rest of the skin though, you could just make a new folder in the Skins folder with at the very minimum a playfield you like better; osu! will use the default settings and images for everything that's missing from the folder.