CSS not being applied to epub

I am using Sigil 1.4.3 to format my ebook. Somehow, the style sheet is not being applied. It's possible that I went through the book and split it into chapters before I added the css file (I discarded my first attempt at formatting, and started over, not sure now if I added the css file before or after), but in any event, the style sheet is not being applied unless I right click each file and click "link stylesheet."

While this works, I would need to do it for every single file in the book. Is there a better way? If not, will adding it manually be OK?

Thank you kindly in advance for your help -

RatherBWriting