Can't set custom theme per space

This is covered elsewhere (see related) but it took me forever to find the information I needed so I thought I'd log this issue for anyone else using the same keywords as me :)

The bootstrap sequence in D7 means that the theme is initialised before spaces overrides the variables, which means that setting a custom theme per space will not work.

I'm using spaces with PURL, and my workaround, in a custom module, was this:

function mymodule_custom_theme() {
  purl_init();
}

Calling spaces_init() doesn't seem to do the job.

I'm offering no deeper explanation or suggestion, just literally wanted to put this here for anyone else searching for an answer to this...