Helpdesk

To edit this Toolbox: More Tools (sharepoint.com)

Sharepoint Admin users can go to this Sharepoint List, to edit or add a new ToolBox:

Intranet - E2 Toolbox - All Items (sharepoint.com)

The icons are based on Font Awesome icons, and the full list of icons can be found here (not all of the icons are supported, so make sure to test this first):

Find Icons with the Perfect Look & Feel | Font Awesome

Once that is done, make sure to update the toolbox in the main menu as well (if you are editing the main toolbox). 

Check this guide (Modify Sharepoint Megamenu | Knowledge Base | Helpdesk (freshservice.com)) on how to do this: