add css rule

HTML

<div id="parent">
    <div id="child" style="display:block; background-color:blue; border-radius:50% / 50%; text-align:center; ">test</div>
</div>

JavaScript

function addCSSRule(sheet, selector, rules, index) {
    if ("insertRule" in sheet) {
        sheet.insertRule(selector + "{" + rules + "}", index);
    } else if ("addRule" in sheet) {
        sheet.addRule(selector, rules, index);
    }
}

// Use it!
var firstStyleSheet = document.styleSheets[0];
//remember to be MORE SPECIFIC
addCSSRule(firstStyleSheet, "#parent #chil0d", "padding:30px; background-color:green; color:white");