Yes, and it is called DOM2 CSS.
Let's say you have a selector like so:
.myP {
/* styles for special paragraph elements of whatever */
}
And also assume that it is in the very first stylesheet on the page (you could always adjust the index number of course appropriately):
var rules = document.styleSheets.item(0).cssRules;
var pClass;
for (var i = 0; i < rules.length; i++) {
if (rules.item(i).type == CSSRule.STYLE_RULE && rules.item(i).selectorText == '.myP') {
pClass = rules.item(i);
break;
}
}
Now, you can just go like:
pClass.style.backgroundColor = 'red';
and it will dynamically modify the style rule, which in turn affects all elements which inherit from it.