In my office we have a build monitor running on a big TV so we can all see whether our software is broken. It's just running a full screen browser pointing to an internal web server. We decided to make it a bit festive and after many iterations came up with the html and images in the attached zip. Basically, it loads the original wallboard display web page in a full screen sized iFrame, and overlays another Div with falling snowflakes.
If you have a similar wallboard display and a similar desire to make it Christmassy, unzip it somewhere and point the browser at the wallboard.html file. Edit wallboard.html and set the url in the bit that currently holds the raspberry pi homepage to the page you normally show on the wallboard. It's fairly near the bottom as the code for the snowflakes is at the top.
A couple of possible issues:
Some web pages don't like running in someone else's iFrame.
This runs pretty smoothly on a Pi 3 with chromium, but I would expect problems on lower spec machines.
This is actually a cut down version of ours, as we have also added a few other Divs full of useful information to the side of our build monitor page.
Hope you fine it useful.