I've reported this problem to the XC5 dev team:
https://bt.x-cart.com/view.php?id=45802
The widget that renders the attributes is this one:
\XLite\View\Product\Details\Admin\Attributes
So, to change the CSS you can "decorate" the class from a custom module or the Custom Skin module and add a path to your CSS file in the getCSSFiles() method.