Yes you can, according to the different choices, you can have the user select the color he/she wants and then you can set up a node.js server to render out a CSS script accordingly.
Check out how bootstrap uses this method to save the file according to the users choice :
http://twitter.github.io/bootstrap/customize.html
Then, rather than making the file downloadable, you can store it in the users directory and have the location of it also stored in the Database and next time, you can use Yii to call for the specified CSS file in the HTML page. This would change the look and each user can have different looks.
Ex :
<link rel="stylesheet" type="text/css" href="<?php echo Yii::app()->request->baseUrl; ?>/css/user/<?php echo $user->name."css"; " media="print" />
Hope this helped. Comment it you need any more information.
Regards,