Modify global CSS with JS

example.js

function css(selector, property, value) {
  for (var i=0; i<document.styleSheets.length;i++) {
    try { document.styleSheets[i].insertRule(selector+ ' {'+property+':'+value+'}', document.styleSheets[i].cssRules.length);
    } catch(err) {try { document.styleSheets[i].addRule(selector, property+':'+value);} catch(err) {}}//IE
  }
}

// Example usage:
css('.header', 'border', '5px solid green');