Here is a handy JavaScript function that changes any element of class oldClass to newClass in the current document. This is especially useful for changing styles on the fly using CSS classes instead of hard-coded style values.

function changeClass(oldClass, newClass) {
    var elements = document.getElementsByTagName("*");
    for( i = 0; i < elements.length; i++ ) {
        if( elements[i].className == oldClass ) elements[i].className = newClass;
    }
}