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/default-setting.js

@@ -16,5 +16,6 @@ module.exports = {
   useKeyboard: true,
   useSelectionScroll: false,
   wheelPropagation: false,
-  wheelSpeed: 1
+  wheelSpeed: 1,
+  theme: 'default'
 };