I updated my comment. I wouldn't approach it from the idea of a php parser. Also, don't assume that its a couple of "shitty themes". I hate to tell you this, but wordpress wouldn't be where it it today without these "shitty themes". Writing a blog engine in node.js is a great start, but to gain traction you need to appeal to the masses.

If you're shelling out to PHP to execute the themes (which is likely to take up the bulk of processing time outside of talking to the database), why write the rest of the CMS in a language other than PHP?

Considering WordPress uses PHP for its templates (like "<?php the_title(); ?>"), it would be more sane to just rewrite the themes to use the templating engine Calipso uses.