FrameMaker 11 - Structured/DITA - Modifications to the Style Sheet Vanish

I need to permanently modifiy the style sheet for the project in which I am currently involved. I can modify the style sheet for each file. PDF it and it looks fine. However, when I save it and reopen it in FrameMaker, the styles revert to the original style sheet. I need to change the style sheet and save it to match this project. How do I do this?