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.
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.
c0ffee, dadb0d, badcab, deface, deadbf, baddad, daddad, debeef, beefed, adbeef, beeeef, efface, faceb0, deeded, facade, 2faced, decafe, dabeef
#debeef is a lovely lavender actually!