what about something along these line, caching and your business model aside....
if(isset($_GET['themeKey']) && !empty($_GET['themeKey']))
$sSQL = sprintf(
"SELECT css FROM themes WHERE themeKey = '%s'",
$rResult = mysql_query($sSQL);
if(mysql_num_rows($rResult) > 0)
$aResultSet = mysql_fetch_assoc($rResult);
Your form would get the required data and a unique theme name from the user and submit this to 'add.php' on your server, your server would then save this to the database. Of course this could quite easily be a local file, or as hinted at, a post-processed theme based on the submitted values.
<form action="http://www.yourserver.com/themer/add.php" method="post">
<!-- FORM FIELDS -->
The user would then just add 'style.php' as the src for a style element on their page, along with defining the RYAN_APP_THEME_KEY constant inside it.
This would require the user to edit this file each time a new theme was created though, if you had a username / password combo though, you could pass these instead and load the latest CSS created.
style.php would obtain the data returned by getCSS.php located on your server and output it as CSS based on the theme key.