Well, you're talking about header and it not working... So I feel like you're referring to Look and Feel -- when you're using Themes (as the default appearance creator-thing).

You can turn off Themes and use Look & Feel to edit the appearance; check out how to do that all here: http://help.xanga.com/topic/i-dont-want-to-use-themes-how-can-i-get-my-look-and-feel-back

Or you can check out other pre-made Themes (and see if you like any of them more) here: http://themes.xanga.com/themehome.aspx?t=r&l=0&pid=1&u=all
You can also try to edit your existing theme...

Well, good luck with whatever you end up doing! :D