Right now, every mouse click or keyboard press is changing the content of the style tag, forcing the browser to recalculate the style:

You can avoid changing the content of style_element on set_css if the new content is the same as the current one.
Since I don't need support for IE, I have used the following:

set_css = function(css_text){
    if (css_text !== style_element.innerHTML) style_element.innerHTML = css_text;
}

However, the code can be changed to allow support for IE by checking the other method as well, or maybe keeping a boolean variable inside the anonymous function