First create the TV. If you want the css files to be selected from a list when creating documents, use "DropDown List Menu" as the Input Type and "css1==css1-document-alias.css||css2==css2-document-alias.css" as the Input Option Values (not including quotes). If you want to write the urls/aliases to the css files by your self when creating documents use "Text" as the input type. You can also set the default value for the tv to the Default Value field.
Then tag the templates what will be using this TV. Then go an edit your template and use the created template variable for the css file location, like:
<style type="text/css">
@import url(’[(site_url)][*name_of_tv*]’);
</style>
Then create a document using this template and you should see the TV at the document where you can select which CSS file to use.. Not sure if this all works, didn’t test it, but it should go something like that