Board Game Arena

How do I give a player a "red thumb"?

Either at the end of a game played with that player (on the left of the game result pannel), either going on the player's profile (under reputation tab > My opinion about), there is are small icons of a green thumb and a red thumb (looking like this: https://imgur.com/a/HuCgBFu). When you click on one of them the other disappears.