This line from the default theme CSS is causing the header to have a background gradient:
CODE
tr.header-top td { background: url('themes/default/images/layout/head_bg.gif') repeat-x; }
If you remove it, the logo will still have the gradient background, so that will have to be changed as well, but you can add this to your own CSS stylesheet:
CODE
tr.header-top td { background: none !important; }
... or in the header html:
CODE
<style type="text/css">
tr.header-top td { background: none !important; }
</style>