code: public String css(String attribute) {
          if (!element.is().present()) {
              return null;
          }
          try {
              WebElement webElement = element.getWebElement();
              return webElement.getCssValue(attribute);
          } catch (NoSuchMethodError | Exception e) {
              log.warn(e);
              return null;
          }
      }
docs: Retrieves the provided css attribute of the element. If the element isn't
  present, the css attribute doesn't exist, or the attributes can't be
  accessed, a null value will be returned.
  
  @param attribute - the css attribute to be returned
  @return String: the value of the css attribute