Are you referring to the text bubbles in messaging? No, you cant change that (unless you jailbreak). You can change your wallpaper on your homescreen and lockscreen to your favorite sports team.
I think you are referring to the wallpaper? If so, go to Settings App/Wallpaper and change it. If you don't like the stock wallpapers you can download a wallpaper app and get new ones from there. I'm sure you can find a Raiders wallpaper somewhere .
If you want to make your icons looks different, you'll have to jailbreak. There are some really nice themes available!! Check out WinterBoard and DreamBoard!