D.G.
Found a Robin costume here:
http://www.buycostumes.com/Teen-Titans-DC-Comics-Robin-Mu...
This link gives ideas for a Batman theme as far as food and activities, just adjust more for Robyn, where you can.
http://ezinearticles.com/?Batman-is-in-Gotham-City,-and-t...
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";
http://www.amazingmoms.com/htm/party_super_hero.htm
HTH