Pretty sure it stands for Colors Secret Santa.
How it works:
You make a picture saying that you're joining Secret Santa (or #CSS). In the picture's description, you write about what you want (as a drawing request) For example: "l want a picture of my characters wearing Christmas hats!" or something like that.
-You will eventually be told to draw for a random chosen user, most likely chosen by UltimateHylian, the host of CSS this year. (Don't tell them you're drawing their request for them! Its a surprise for everyone who's in it!)
So basically its like a gift exchange, but nobody knows who they're drawing for, or who's drawing a picture for them.
Sorry l made that really confusing, just look at the first picture in #CSS with the lil elf guy.

@Flippersurprise - Hylian... xD But thanks for explaining. The only thing you had wrong is that, everyone knows who they're drawing for and who's drawing for them, I will tell you on your css pic :]

^ Wait if we know who's drawing our picture then why is it called secret santa lmao
thats usually not how it works lol

so basically, it's secret santa but just with the colors in front?
i might join maybe

Here in Australia we call it Kriss-Kringle. You don't know who's drawing for you, but you know who you're drawing for.

@flippersurprise - yeah xD That's how it went last year and it's a whole lot easier to do it this way

thanks for telling me :D
i might join

I thought CSS was for Counter Strike: Source. :/