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.
dest doesn't already have a `/` at the end of it? I wonder if it would be easier to just make an entire array of the whole path and use the join method there. For example:
```javascript
var destSegs = dest.split["/"];
var srcSegs = src.split["/"]; 
srcSegs.splice(-2, 1); // remove the second last element to move up one level
return destSegs.concat(srcSegs);
```

You'll probably also have to make sure that dest and src don't contain common path segments.