You can add a Christmas snow effect to your WordPress web site by using a snowstorm javascript which has been around for more than ten years.
This used to be bundled with Jetpacks modules and I believe you can only access it now if your site is hosted on the main WordPress.com, but never fear you can add it to your site with a small snippet of code.
First download the latest version and move snowstorm.js in you js folder in your theme, there is also a minified version that you can use, but you may wish to change a few variables in the script.
Then in your functions.php add in the snippet below (minus the opening php block)
