While the header is an image, you can change it to be a solid colour using CSS - you just won't get the wavy design. In my game, I've changed the header to be black, although you can make it anything you want by using the #status selector.
<![CDATA[<style type="text/css">
#status {
background: #333;
border: 1px solid #333;
}
</style>]]>
Add the code above into a new attribute somewhere (something like game.css would be appropriate) and run a script on startup that prints the attribute, i.e.
msg (game.css)
There is more information on the wiki about modifying the display if you need it. You might have to add the attribute using the code view rather than the attributes tab in the GUI, (I can't remember if this is necessary or not).
EDIT: I forgot to mention that the msg (game.css) script needs to be added to the default function InitInterface (not the actual start script), otherwise it won't work when a saved game is loaded.