Refactor Legacy and Modern Theme Generation

When we started this project we had to support IE11 which influenced what is now the "legacy" implementation of theme generation. When we dropped IE support we switched to the modern style.

These generators currently live in the same file and tend to have large individual functions, where some logic could probably be broken out.

If you're interested, I'd be happy to review a pull request to split out the implementations into separate files and clean up some of the functions and comments.

Make sure all of the tests still pass!