Let it Snow!
As add-ons are programs downloaded from the internet, they are potentially malicious. You should only download add-ons you trust.
To download this add-on, please copy and paste the following code into Anki 2.1:
If you were linked to this page from the internet, please open Anki on your computer, go to the Tools menu and then Add-ons>Browse & Install to paste in the code.
This add-on previously supported Anki 2.0. You can download the last version supporting 2.0 here. As Anki 2.0 has been discontinued, no support is available for this version.
All Anki 2.1.x Add-Ons
Edit: Thanks for the reply and figuring things out! I apologize for not giving a very helpful report at first. I changed my vote to a thumbs up.
However, I did note that Night Mode no longer seems to be compatible with Let it Snow – likely due to updates introduced to the former over the past few months. So until that is resolved – either on Night Mode's side, Anki's side, or perhaps this add-on's side – you might want to disable Night Mode's styling of the reviewer (though I'd assume that having Night Mode is more important than a touch of seasonal ambiance!).
For future reference: It's both quicker and easier to resolve bugs like these when you report them through the bug tracker (see the "Support" section in the description above, or this link: https://github.com/glutanimate/anki-addons-misc/issues/new). Just saying that an add-on does not work for you unfortunately doesn't give me a lot to go on. I do still appreciate the report, though!