You can't just change the folder icon with a setting.
You have to edit the icon itself.
Have a look in /usr/share/icons until you find the icon you want to change, then edit it using an svg editor like inkscape.
To find out if the theme you are using uses any other themes, open /usr/share/icons/Mac4Lin_acqa/index.theme in a terminal and see what it inherits (if anything). This will tell you where the folder icon is. Then find and edit the icon (it will be under scalable/places. You will need to be root to copy the icon somewhere first and make it owned by your user.
When you have done your editing, copy the icon as root back to the right place and give it the same name as the folder you wish to change. There are lots of versions.