FLUID-5482: Using stylus to generate stylesheets required by preferences framework

http://issues.fluidproject.org/browse/FLUID-5482

Note that the manual test manual-tests/framework/preferences/assortedContent/index.html doesn't work with the stylus generated theme files. 

This manual test was designed to test the coordination between prefs editor and various FSS elements. However, Justin is working on FLUID-5469 (http://issues.fluidproject.org/browse/FLUID-5469) to remove FSS from infusion. So the contrast theming for FSS elements is no longer handled in this pull request. Also, In Justin's branch, this manual test has been re-written using html5 elements rather than FSS. Once Justin's work gets in, the broken manual test should be fixed itself.
@acheetham, both "grunt" and "grunt custom" carry out the stylus build automatically. The user doesn't need to run the stylus build first.