So, I have worked up a html menu with a css3 hexagon grid that I'm adding to a page with a custom html module. Can I add a separate style sheet for this module? If so, how?
I agree with @Mikan although if you really want to add a separate stylesheet there are at least a couple of ways to do this:
You don't have to add separate CSS file. You can just style-up new selectors you want, or use some existing in main
custom.css, whichever you are using, and reference them in Custom HTML module.
That also goes along the line of general guidelines of using as small naumber of external CSS/JS files as possible, in order to reduce number of HTTP requests.