There are too many variables to guarantee we could read a custom stylesheet created (or updated) outside of Sculptor. There is no guarantee the CSS code is even valid, and there are complex things you can do such as linking in multiple stylesheets, cascade properties, use hacks, use different media types and things like that. It would be very complex to know exactly what the users intent was with their hand coding.
In your case, I would recommend always saving a preset in Sculptor. That way when you get to step 6, you can load up your preset as a starting point, then update what you need to into Sculptor. Once you have generated the new page, you would have to update your template with the changes which would then update all of your pages accordingly.