It's looking like they're just going to make it a weekly event to motivate people to participate in the Battledome. So if you didn't get a thieves' site theme/avatar, there will probably be another chance down the road. I actually don't think it matters what faction you join, because if they keep rotating it every time, eventually everyone's gonna get all the faction themes/avatars. The main goal probably is just to keep people participating in the new Battledome.
That's the rumor. I saw a petpage that had listed the amount of 'points' each challenger/difficulty level was worth. I assume you need to reach a certain number of points to receive any of the spoils.
Wouldn't the challenger/difficulty matter as well? I would imagine beating Kanrik on Hard 5 times is not worth the same as beating one of the easiest challengers on Easy 5 times.
I did 20 wins on easy for the lowest opponent of each faction (so 40 in total) using the AutoBattler, and that was enough for getting the site theme/avatar/boon. Hope that helps?