Creating animated keyframe background

[achello]

Hi, I'm currently looking to implement a Christmas themed background to our company intranet page. The problem herein lies with the ability to edit the stylesheet. I only have access to the "design" stylesheet (the CMS is hosted by an outsourced company). I've managed to find some useful information online as to how to implement the background but I can't seem to find a way to have 3 transparent backgrounds animate over the original background image.Starting to think this isn't going to be possible!I've attached images for reference points as well as the CSS file. Any help would be great. Scott (Might be worth noting I'm self taught so anything that looks bad, probably is and thus I'm open to suggestions to improve)

snow CSS.txt