There are many, many themes available (free and paid) plus there is a tweak called BytaFont that will allow you to change your phone's system fonts with an almost unlimited selection.
However, you have to be jail broken to change the appearance of your phone. Then, you can access Cydia (the jail breaker's "app store" to browse and get themes and such).
You can't change the phone's appearance (except the wallpaper) if it's stock ... it must be jail broken.
Marilyn