Thanks, everyone!
I've managed to sort it. I just deleted and redid the stylesheet and relinked it, and this time it worked. I think it may have the fact that, as Kevin mentioned, there was no ../Styles/ on that first page. There *was* on the intro page before it that I removed when I was creating the sample, so I'm guessing that only the first section/page (which was an image) linked to the css and the rest of the doc didn't.
Thank you so much for taking the time to help me. You're all rock stars