Layout options to test:

 stylepress outer + stylepress inner
  = good, we nuke all theme styles and replace template_redirect with our own, working fine.
 stylepress outer + theme inner
  = tricky! we hook into compatible themes (e.g. oceanwp)
  we get a start/end hook (e.g. ocean_before_main & ocean_after_main )
  we capture all output between these two hooks using ob
  on the last hook (ocean_after_main) we call our outer stylepress rendering and wrap the already rendered inner content into the outer bit. phew!
 theme outer + stylepress inner
  = tricky as well!
  we get two hooks (e.g. ocean_before_main & ocean_after_main )
  on the first hook we render our stylepress output
  then start output buffering and remove everything we capture until the ocean_after_main hook runs.

 theme outer + theme inner + stylepress elements (modal) added to page
  = we just have to make sure things like modal get rendered without our usualy header/footer hooks.



Themes to test: