Adding Background Image to Winter Style Sheet - Seasons Theme


I'm interested in adding a background image to the CSS of the Winter Style Sheet for the Seasons Theme. I have a vague understanding of where to add the background image code and have tried a series of locations within the style.css folder and cannot seem to find the right location. I have tried adding the html code to the body tag under the General HTML settings as well as the CSS code within the Winter theme settings. Perhaps I need a break from this (ha-ha) but if anyone can point me in the right direction, it is appreciated!

This is the line you want to add your background-image: url(); to.