I wouldn't go that route.
It's not just a matter of applying a different skin. Squirrelmail only knows about themes, which in essence are just color table variations.
The themes you can find on the internet (
http://nutsmail.com/ /
http://www.voxthemes.com/squirrelmail.php ) actually tap into all of SquirrelMail's source pages. In other words they modify SquirrelMail per se. This also means that you will need the theme supplier to supply you with an updated version of Squirrelmail each and every time there are changes.
What you can do is choose a color scheme of your liking and do a few adjustments of your own. It won't look like
http://www.voxthemes.com/squirrelmail.php , but if chosen wisely it will look good enough.
Alex