Change CSS rule at runtime

Question

How do I change any CSS properties during runtime?
Suppose I have:

#mycss {
   background:#000;
   padding:0;
}

Then during runtime,

Answer 1

Add jQuery to your page. Then:

$(function() {
    $('#mycss').css('background-color', '#FFFFFF');
    $('#mycss').css('padding', '10px');
});

Look up the documentation for the .css() method.

But yeah, no one really knows what you mean by "runtime".