themeable perfect-scrollbar

- added „theme” parameter
- replaced all the CSS visual styles with SCSS variables
- added $ps-theme-default SCSS map that holds all the variables
- added $theme parameter to all the mixins

src/js/plugin/initialize.js

@@ -26,6 +26,7 @@ module.exports = function (element, userSettings) {
   var i = instances.add(element);
 
   i.settings = h.extend(i.settings, userSettings);
+  cls.add(element, 'ps-theme-' + i.settings.theme);
 
   clickRailHandler(element);
   dragScrollbarHandler(element);