Javahelp is the Java based help system that can easily be integrated in a Java application. The documentation does not give any information about it. On the web, many people state that it is not possible, that the help browser always uses a built-in font. I am not convinced that it is not possible.
The help system of the IntelliJ IDE is based on javahelp and uses another font. So I think it must be possible. Is there someone who actually did this?
For completeness:
Stylesheets seem to work ... in certain circumstances. Probably only a subset is supported, and the trick is to find out which one.