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

First of all, CSS should be inserted as "titled" link:

If I will to change in this CSS next selector (no matter what [ctr-panel~="button"] means):

[ctr-panel~="button"] {
    width: 25px;
    min-width:  25px;
}

To do so in JS write:

    for (var i = 0; i < document.styleSheets.length; i++) {
        var sheet = document.styleSheets[i];
        if (sheet.title == 'fillCtrl'){
            for (var j=0, n=sheet.cssRules.length; j < n; j++){
                var rule = sheet.cssRules[j]
                if (rule.selectorText == '[ctr-panel~="button"]'){
                    rule.style.cssText="width: 12px; min-width: 15px;"
                }
            }
        }
    }

Obviously, it might be another logics to find appropriate rule.style.