Odd renderer behavior when choosing a new theme

When using the theme selector to choose a new theme on the site, we have observed some odd behavior where the new theme will display, but uses the prior theme's renderers. Not entirely sure how this happens or it might be specific to our themes. I suspect that something gets setup in the admin_externalpage_setup('themeselector'); call that doesn't get updated when a new moodle page is created. Either way, I think the provided patch is a good improvement, removes some odd looking code and the new theme is selected throughout the request.