public function getPaneCssClass($index = null, $label = null)
    {
        if (is_string($this->paneCssClass)) {
            return $this->paneCssClass;
        }

        if ($index !== null && isset($this->paneCssClass[$index])) {
            return $this->paneCssClass[$index];
        }

        if ($label !== null && isset($this->paneCssClass[$label])) {
            return $this->paneCssClass[$label];
        }
    }