Jokes aside, the designers ran out of time to create a "Welcome to Stack Exchange" banner. We wanted to throw as many 90s website tropes in as possible on a very short schedule. On Friday morning we considered whether to ship the theme to just Stack Overflow or to ship it everywhere with the wrong banner. I advocated for keeping to our original plan and vowed to lampshade the problem in my meta post announcement.
Ultimately the goal of an event like this is the same as Winter Bash: remind us that participating on the network is supposed to be fun. No, it's not unicorns and flower trails all the time, but asking and answering well is one of life's great pleasures. Goofy, dumb events like this contrast with the rest of the year not because we aren't having fun, but because our enjoyment comes from achieving a good purpose rather then novelty. It's obviously important on Stack Overflow, but it's no less importance on other sites. So while it would have been easier to just write one post for Meta Stack Overflow, I pushed for us to make the feature available everywhere.