The main.css file it imports dates itself to March 9 of 2005, and is housed in an "ancient history" section of the website that covers everything before October 26, 2010, so: "sometime between those two years" =P
Its first appearance on the WayBack Machine is October 13, 2009, which narrows the range somewhat.
Its first appearance on the WayBack Machine is October 13, 2009, which narrows the range somewhat.