I don't understand how... You always have two thumbs there which means you can contrast the state of being just an outline with the state of being filled in.

It happens to me all the time. I can’t explain it either, but my hypothesis is me interpreting the highlighted “like” button as the encouraged action out of the two?

That makes sense now that the other reply pointed out the Join button being filled in. The like and dislike buttons are internally consistent but the entirety of the buttons down there are not consistent.

And yet the "join" button is filled in without being joined. Let alone, the icons on the video player which have their own varied little philosophies

Huh... somehow I noticed that but it never really registered with me that it's inconsistent because I've never wanted to join a channel.