Found a Robin costume here:
This link gives ideas for a Batman theme as far as food and activities, just adjust more for Robyn, where you can.
Maybe make it a SuperHero party...with him being Robyn and everyone else can come as who they want (but no other Robins), maybe a Teen Titans party or Justice League Party. Your colors could still stay the same, there are tons of "Gotham City" backdrops to add to the look as well.
Here is a link to Justice League Party and Decor ideas including a "Hall of Justice";