Hi Katie,
In your skin.htm file you do not need to specify the CSS file that you are linking to, it automatically is set within DNN. - You should add your styling to the skin.css file.
There are other css files in DNN and you can read about how they all work here:
Introduction to CSS and inheritance in DotNetNuke skins
I would suggest you go to our tutorials page and look at the skinning section, if you follow the tutorials through in order it will save you loads of time.http://www.dnncreative.com/Tutorials/tabid/261/Default.aspx
Thanks,