Just use a simple stylesheet setup. Example code (albeit not very thorough):
$skin = isset($_GET['skin']) ? $_GET['skin'] : "default";
echo "<style type=\"text/css\" media=\"all\">@import \"".$skin.".css\";</style>";
That first line checks if $_GET['skin'] exists and if it does, uses that stylesheet. Otherwise it would use 'default' stylesheet. BTW, you'd place that somewhere in the <head> area of your document.