Convert all Sass files to be partials

We are currently generating several CSS files from our Sass files. The maintainers are using those generated CSS files as the basis for the vanilla CSS files used by themers who aren't using Sass, but those generated CSS files are completely useless for the themers who are using Sass.

In my last project, I converted all of the Sass files to be partials and it worked really well. I was able to remove all the @import "base" lines from the tops of the Sass files too.