Fix stylesheet portion of bower.json to comply w/ new `main` spec

Per https://github.com/bower/bower.json-spec/commit/0b34d2bb0bdd4fa31fa9728f787f49e170e41f99
> * Use source files with module exports and imports over pre-built distribution files.
(See also the example given in the spec; the CSS files aren't included, but the main SCSS file is.)