Honestly, taking a snowing effect randomly on the Internet and trying to make it work on Discourse isn’t a necessarely easy task.
Discourse has its own constraints, and most javascript from the wild that you want to include in Discourse needs to be modified and adapted to these constraints.
I tried yesterday and in the current state, the effect is quite… Subtle. It surely could be improved, but again, that’s some work.
I also agree with the others, if you want to add customizations to your website (you seem to be very involved in it and read Meta a lot), you should start by the beginning and understand the basics of HTML, CSS and javascript.
I think free help here for such customizations will be limited.