Add the CSS file as a resource to your project so it can be published with the project on deploy.
As far as IncludeHTML comp, to try and use it with this scenario, you probably just need to put a LINK tag in there. LINK tags look like this:
<LINK rel="stylesheet" type="text/css" href="<NAME OF CSS FILE>"></LINK>
You'd want to add this as text vs. a URL.
I'm testing this out now to see if it works. But the more elegant solution would be to use a theme (Workflow's application of stylesheets). You can use one of several themes that come with the product or create your own in the Theme Editor (which will also convert a CSS to an xTheme, if you need it). Then you can just flip the theme on the form.