So what is the resolution to this?

Sherif, we're applying custom CSS in spaces and globally already, which is specifically why we are asking for this functionality. Your suggestion ignores the scope of the request: we do not want to change the default behaviour/styling. Of all the built-in macros, the Panel macro is the one where this functionality seems to be most useful.

An example: I want to assign a class of, say, 'bluebox', and make the panel blue, through either space-level or global stylesheets.

We're not asking for "style='blah: blah'" fields for users to do their own whole stylesheets, just class assignment, which (to me) seems to balance smartly between functionality and security. We want this because we need to classify different sets of information, similar in concept to the 'tip', 'warning' and 'note' macros classify those sets of information. The current in-house practice is to assign specific colour values to each macro, which is tedious and mistake-prone.

I wanted to avoid it, but I guess I'll be forced now to create a host of new user macros or rewrite a new Panel macro to work around this issue.

Thanks for the feature request, Aaron. In general, we don't really want to add a CSS field to all our macros so users can style them.

We've got a more general solution available - you can apply custom CSS to a space and/or a whole Confluence instance. I'd recommend that approach. You can learn more here: https://confluence.atlassian.com/display/DOC/Styling+Confluence+with+CSS