I was afraid of installing this module because it hasnt been updated long time ago and it stil at version 1.0 but it does the trick. :) Other modules I tried could not separate the template css from mine in the module. This has this option and you can input your custom css independently. It wont broke the module. I would give a 5 star but the "HTML Header Options" doesnt work for me. When I saved the stylesheet link rel in the HTML Header Options area the area becomes always empty after saving. :(
Thank you very much for your feedback!
The parameter you mention wasn't designed to accept file names or link tag, it was accepting style information.
Since you and other people might need to reference an already existing .css file, additional parameter was added for that in 2.0.