injectCss.js

(function () {
  var newcss = '#myDiv { -webkit-filter: blur(8px) grayscale(1); }';
  newcss += '#hideSomething { display: none; }';
  newcss += '#makeItGreen { background: #00ff00; }';

  if ('\v' == 'v') /* ie only */ { 
    document.createStyleSheet().cssText = newcss;
  } else {
    var tag = document.createElement('style');
    tag.type = 'text/css';
    document.getElementsByTagName('head')[0].appendChild(tag);
    tag[(typeof document.body.style.WebkitAppearance == 'string') /* webkit only */ ? 'innerText' : 'innerHTML'] = newcss;
  }
})();