add(name, contents) {
    var theme = this._themes[name];

    if (!theme || !contents) {
      return;
    }

    if (theme.url) {
      contents.addStylesheet(theme.url);
    } else if (theme.serialized) {
      contents.addStylesheetCss(theme.serialized, name);
      theme.injected = true;
    } else if (theme.rules) {
      contents.addStylesheetRules(theme.rules, name);
      theme.injected = true;
    }
  }
Add Theme to contents
@param {string} name
@param {Contents} contents