Use url_third_themes & path_third_themes

The location of the third-party themes directory is user-configurable, so
it is not guaranteed that the third party themes will be installed in
{theme_folder_path}/third_party. Use the configuration variables that
point directly to the third-party themes folder to work with systems that
have configured their third-party themes to be in different locations.
Merge pull request #5 from bryanburgers/third_themes

Use url_third_themes & path_third_themes