I don’t know how long these options have been there, but the View menu in Messages for Mac has some useful toggles. “Show All Buddy Pictures in Conversations” will add some much-needed personality to chats and make group chats much, much easier to follow.
“Use Alerts in This Conversation” (enabled by default) allows you to mute individual conversations instead of turning on system-wide Do Not Disturb feature. This feature has a strange name, however, since it’s the same as right-clicking any conversation in the list and enabling “Do Not Disturb.”