Here’s how you get to a page of icons that can be used for UI macros and field decorators in Servicenow UI 16:
https://[instance-name]/styles/retina_icons/retina_icons.html
credit to this answer on the community https://www.servicenow.com/community/developer-forum/using-different-icons-for-ui-macros-fuji/m-p/1909528