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');
            var bgimage = th.css('background-image');
            if(bgcolor != 'Rainbow' && bgimage == 'none'){
                var Pc = bgcolor.match(/rgba\(\s*\d+\s*,\s*\d+\s*,\s*\d+,\s*(0?\.\d+||[01](.0*)?)\s*\)/);
                Pc = Pc && Pc[1] ? Pc[1] : 1;
                Pc = Math.round(238 * Pc);
                var grayscale = 'rgb(' + [Pc, Pc, Pc].join() + ')';
                th.css('background-image', 'linear-gradient(to bottom, ' + grayscale + ', ' + bgcolor + ')');
            }
        });
    }
});