If you have a predefined CSS styling e.g. in your template for a button by a class, you can easily enable it for a layer of the slider by add this class for that. Or in the other hand, you only need to define a style once for a class, then add it for more elements easily.
Click for edit the slider, then select a layer. Then click on the layer's Links & Attributes tab, and add the desired class into the Classes parameter. You can add more classes here, just separate them by space. You can learn more about the other options there in the Attributes settings article.