Is it possible to add CSS to theme without a subtheme?
My understanding is to never use hook_init if possible. It's a hook that's call in the early stages of the bootstrap process and can produce some funky results. The API docs clearly state:


  To add CSS or JS that should be
  present on all pages, modules should
  not implement this hook, but declare
  these files in their .info file.


I'd suggest adding a stylesheets[all][] entry in the .info file -> Adding style sheets