Just for clarity's sake, let me make sure of some term definitions?
theme = on the theme list page, the "themes" are the things with the numbers in front of them. (ie, pirates, cowboys, etc.)
category = the heading of each section of themes? (ie,"meta", "cosplay", etc.?)
"subject" = combination of pairing and...category? I had to stare at this a long time to figure it out.
And, to clarify, cuz it's late and i r dum... PER PAIRING: a) only one person can claim a given category-wide challenge. BUT b) unlimited people can do mixed bag challenges. (ie, only one person can category-claim "Meta" for Archer/Kimberly at a time, but unlimited folks can do Archer/Kimberly mixed-bag challenges at once) ...Yes?
oh man you make me wish I didn't have four million things to do. :tells challenge to wait a few months: ._.