As always, when something takes me a while to figure out, I try to post the generally useful technical information on my blog. For the new copyleft.org site, I've been trying to get all the pages branded properly with the header/footer. This was straightforward for ikiwiki (which hosts the main site), but I spent an hour searching around this morning for how to brand the GNU Mailman instance on lists.copyleft.org.
Ultimately, here's what I had to do to get everything branded, and I'm still not completely sure I found every spot. It seems that if someone wanted to make a useful patch to GNU Mailman, you could offer up a change that unifies the HTML templating and branding. In the meantime, at least for GNU Mailman 2.1.15 as found in Debian 7 (wheezy), here's what you have to do: