I have a custom style sheet file that I've applied to all of the sites
in a single site collection. I also have a custom master page loaded as
a feature on the server. The custom master page does little other than
to incorporate the custom style sheet and to add a custom search box to
the side bar.

The only trouble is that all of the _layouts page use
application.master, rather than the style sheet I specified.

So what is the best practice to incorporate the style sheet into the
site collection, including the _layouts pages? Editing core.css,
application.master, and any pages in the layouts folder on the server
are all no-nos. Is there a supported alternative, or do you have to
violate one of the no-nos in order to do it?