There are two methods to save kits to your Favorites in Arcade: From Arcade's Browser, and from Arcade's Play page.
Managing Favorites from the Browser
To favorite or un-favorite a Kit from the Browser:
Hover over the Kit
Click the ellipsis at the bottom right of the Kit
Click Add to favorites
Managing Favorites from the Play page
To favorite or un-favorite the kit you are using currently from the Play page, click the Heart icon next to the Kit name. Favorited Kits will display a filled-in Heart icon.