LESS is much more maintainable than CSS, and having the styles and themes in LESS would also make them really easy to tweak on systems rather than having to rely on an external tool (them roller).
Someone has already taken the time to do this, although I don't know if it is completely synched with the latest jqm or not: https://github.com/srsgores/jquery-mobile-less
Also you can generate CSS from it easily so it shouldn't be a compatibility problem.
Bonus points for any compatibility with Twitter Bootstrap LESS :-)
Thanks for your post.
Please, see the thread of ticket #453 about SASS/SCSS and LESS. Although that ticket is closed the discussion is still alive, so I am going to close this one as duplicate.