Custom css with webhelp

I want to use a custom CSS file with webhelp output. I'm using Oxygen. I set the html.stylesheet parameter and pointed it to a custom CSS file. 

In my output, however, the CSS is not linked. I only see one <link> for css in the output, which is for tocWDiv.css.

Does the html.stylesheet parameter not work the way I'm expecting it to with webhelp?

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David Goss, M.A. 
Technical Writer, Laboratory Division 
Frontier Science & Technology Research Foundation 
4033 Maple Road 
Amherst, NY 14226 
(716) 834-0900 x7218