I am wondering is there any retargeting codes embed in Itch.io website? Like those codes that get whoever look at some games in Itch.io but didn't purchase anything. Then that person goes to another web and the game they previously visited in Itch.io appears on that web's ads.  

Also, it is possible to install Facebook Pixel for my own games in Itch.io? 

You can add third party analytics from this page: https://itch.io/user/settings/analytics

We don't do any retargeting ourselves, but it should be possible to implement once you add the Facebook pixel.