MediaWiki:Gadget-GradientHeaders.js

$('tr').each(function(){
    var tr = $(this);
    if(!tr.hasClass('rainbow')){
        tr.children('th').each(function(){
            var th = $(this);
            var bgcolor = th.css('background-color');
            th.css('background-image', 'linear-gradient(to bottom, #eee, ' + bgcolor + ')');
        });
    }
});