Theme enhancements

Two main features here:

1) Put the frontpage under the themes section
2) Add a method to ApplicationHelper called `theme_partial`, so the `theme` variable isn't hanging around, and is neatly tucked into the method

(1) is only there to help make room for my `wsbooks` folder that I have yet to create.

(2) helps to make it clear that certain elements are from the themes folder, and helps encourage further theme development.