2
In your theme you should have these three lines...
<link rel="stylesheet" type="text/css" media="screen" href="<{$xoops_url}>/xoops.css" />
<link rel="stylesheet" type="text/css" media="screen" href="<{$xoops_themecss}>" />
<{$xoops_module_header}>
The first line is the default xoops css, the second line is the theme css and the third is loading the file header.php from the module root directory.
Personally I think the theme CSS should be the last one listed...
You could do some things in the header.php file and perhaps do what you want but I believe the browser will process any CSS files in the order received and if you include a CSS sheet in your header.php file it would overload any other properties from the other two CSS sheets. You might be able to play with the order of these or check for properties with the header.php file and go from there.
Rodney