Make sure the snow represents only 7% of the theme. That's what it is across the country right now compared to about 32% last year. Global warming is upon us all.
Got it to snow a little bit on the "Christmas" theme. There are "Snow Effect" selections at the bottom of the main index page that change the way it snows but they don't hold beyond that page. I might figure out how to make it work well before Christmas.
vBulletin has designed this stuff to make it all but bulletproof to changes unless you're one of "them". About all a novice can do is break it - and I'm good at that.