Skins allow various layouts with just CSS. Some options below depend on a skin. However a combination of skins and options may lead to unpredictable layouts, get yourself dirty. E.g.: Skin Split requires any split layout option. Failing to choose the expected layout makes it useless. See <a href=":url" target="_blank">SKINS section at README.txt</a> for details on Skins. Leave empty to DIY. Or use hook_slick_skins_info() and implement \Drupal\slick\SlickSkinInterface to register ones.