RoyalSlider skin is a CSS file with set of styled controls, it can be added to any type of slider.
Inside of royalslider folder you'll find three other folders - default, default-inverted and minimal-white, these are folders with slider skins. They include: .css, .png and optionally .psd file. Each skin has specific CSS class name that you should add to slider (open CSS skin file to find its class name). For example to select minimal-white skin rsMinW class should be added.
Skin CSS file is structured and commented, you're free to modify it however you wish.