Actually, you can create a CSS rule that will affect all elements on the current page. In most browsers it should be as simple as:
var style = $('<style>body { background: green; }</style>')
$('html > head').append(style);
This may or may not work in IE, however you can use IE’s proprietary addRule instead:
document.styleSheets[0].addRule('body', 'background: green', -1);
Naturally this will not assist you in creating css files that can be shared between webpages, but it is a handy way of affecting the style of a large number of elements without the need to iterate over them.