You can add buttons to menu items, buttons, and some sections that support it.
To add an icon to a button:
Also, you can search for some specific icon and add the link to it. Find the Search field in the Select icon window.
You can add icons to the menu and submenu items.
You can’t add icons to site sections freely. It is possible only with sections that already include icons by default.
Unfortunately, it isn’t allowed to add custom icon fonts this time.