I have the Green Sun Sparkle Avatar Border also from the quest-a-thon. What are these, are they wearable?

They show a border around your HA face. Like mine ( Green ). You can activate those you have unlocked here:

https://new.subeta.net/cosmetics/avatar_borders

So you go into your wardrobe and hit 'personal' at the top banner. It's under cosmetics in the menu that pops up.

Or you can just click here :'D

https://new.subeta.net/cosmetics/avatar_borders

They're used in news posts!

Ah, thank you