[j0hnik]

(function(selector, options) {
      var style = document.styleSheets[0];
      var rules = style.rules;

      [].some.call(rules, function(rule) {
        if (rule.selectorText == selector) {
          for (var prop in options) {
            rule.style.setProperty(prop, options[prop]);
          }
        }
      });
      
    })('.test', {
      'border-bottom-width': '600px', 
      'border-left-width': '500px'
    });

пригодится