You can change it in your profile settings (though that’s not exactly what you’re asking for). I think there’s a karma threshold, though.
c0ffee, dadb0d, badcab, deface, deadbf, baddad, daddad, debeef, beefed, adbeef, beeeef, efface, faceb0, deeded, facade, 2faced, decafe, dabeef
#debeef is a lovely lavender actually!
For anyone looking for a more tasteful topcolor, try d0c8b5. It's just a darker version of the main HN background color, so it's easy on the eyes.