Hey guys, you would only see the xmas theme on sign-in if you have changed your default view of the site in your profile.
I know some of you did due to the colours last time.
Claire, if you would like the old one back for now:
1) Goto your 'profile', the button is at the top of the page
2) Select the section marked 'Look and Layout Preferences'
3) At the top where it says current theme you can change that to the original or something else you fancy

The xmas theme will stay for now, though for the new year I'll see if I can track down a nice sunny one to look forward for...or just go back to the good ol' reliable one

PS: Merry Christmas!