Hello Kirsty Hay,
Thank you so much for the question regarding our theme Corporate Plus Pro. You can get the numbers of icons from font awesome icons. If you want to add your own icons, you need to customize the theme because there is no any such options to add your own custom icons. If you are familiar with WordPress codes you can customize the AT Service widget code and use your own custom icons.
Note: Child Theme is recommended to customize theme.
Regards
Acme Supports