Custom Style Editor is a quite useful part. You can define your custom style definition for any element, but it is recommended to use this tool only related for your sliders. You can also define the Side color of 3D objects, which has point in case of a 3D transition between the slides. The default value is #777, but you can change that for any custom one, for example if your site has a white background behind the slider, then it is recommended to set it to #fff value.
If you make any changes make sure you press the Save button. Any of these modification will be kept until you remove that, so updating the Layer Slider has no effect for this.