code: public void setFontStyle() {
  
          if (!CmsStringUtil.isEmptyOrWhitespaceOnly(m_styleName)) {
              removeStyleName(m_styleName);
          }
          String id = new CmsUUID().getStringValue();
          m_styleName = CSS_CLASSNAME + "_" + id;
          addStyleName(m_styleName);
          String js = "var elements = document.getElementsByClassName('"
              + m_styleName
              + "');"
              + "var iframeContainer = elements[0];"
              + "var iframe = iframeContainer.getElementsByTagName('iframe')[0];"
              + "var iframeBody = iframe.contentDocument.getElementsByTagName('body')[0];"
              + "iframeBody.style.fontFamily='\"Open Sans\", sans-serif';";
  
          JavaScript.getCurrent().execute(js);
  
      }
docs: Sets the font stype to Open Sans.<p>
  Has to be called after every refresh of the UI. For example after switching tabs on tab-sheets..<p>