There are a few different reasons for the skins. The first is obvious, they are fun. However, there are more serious reasons for skins. A few of the skins provide large text, for people who have a hard time reading smaller text. Some of the skins are designed so that POPFile will display as much information on one screen as possible. You may also find some of the skins are easier to read on a laptop or a CRT. They are there to do what you wish to do with them.
Skins are created using CSS. With a little knowledge of CSS, you can use existing skins as a basis to cusomize POPFile's UI as you wish. Starting with POPFile 0.22 skinners have even more power to make POPFile look the way they want through the HTML templating system.