scooby-doo.user.js

// ==UserScript==
// @include http://*/*
// @include https://*/*
// ==/UserScript==

//applies a "main-content" style to the first top-level element that isn't
//a header, nav, aside or footer

var style = document.createElement('style');
style.innerHTML = ".main-content {outline: 5px dotted deeppink !important;}"
document.body.appendChild(style);

var kids = [].slice.apply(document.body.children);

for (var i = 0, l = kids.length; i < l; i++) {
	switch(kids[i].tagName.toLowerCase()){
		case "header":
		case "nav":
		case "aside":
		case "footer":
			break;
		default:
			kids[i].classList.add('main-content');
			break;
	}
  //comment out the following break if you want to apply main-content to all
  //non-header/nav/aside/footer elms which doens't really make sense.
	break;
}