colibris: introduce skin variants, in order to customize the rendering

This provide a nice way to change the colors of main containers from settings file. See comment inside settings for how it works

settings.json.docker

@@ -95,6 +95,30 @@
    */
   "skinName": "${SKIN_NAME:colibris}",
 
+  /*
+   * Skin Variants
+   *
+   * For the colibris skin only, you can choose how to render the three main
+   * containers:
+   * - toolbar (top menu with icons)
+   * - editor (containing the text of the pad)
+   * - background (area outside of editor, mostly visible when using page style)
+   *
+   * For each of the 3 containers you can choose 4 color combinations:
+   * super-light, light, dark, super-dark.
+   *
+   * For example, to make the toolbar dark, you will include "dark-toolbar" into
+   * skinVariants.
+   *
+   * You can provide multiple skin variants separated by spaces. Default
+   * skinVariant is "super-light-toolbar super-light-editor light-background".
+   *
+   * For the editor container, you can also make it full width by adding
+   * "full-width-editor" variant (by default editor is rendered as a page, with
+   * a max-width of 900px).
+   */
+  "skinVariants": "super-light-toolbar super-light-editor light-background",
+
   /*
    * IP and port which Etherpad should bind at.
    *