[development] Theme renders as tokens in 4.6

[Rob Thorne]

I'm trying to code a module that sets the theme for a class of users by 
(1) using HTTP BASIC AUTH, and (2) changing the theme according to what 
BASIC AUTH user they log in as; this is used in a two tier login system.

This is a 4.6 site, and I'm trying to do what og.module did in 4.6 
time:  set the global $custom_theme to the name of the theme I want.  
I'm doing this in my menu handler in the uncached section of the handler.

The first time someone calls a theming function, init_theme() gets 
called, and init_theme is seeing the right value for $custom_theme.  But 
if I select anything but the current default theme, the page renders in 
a very weird fashion:

    {head} {styles}
    {_BLOCK_.header.logo} {_BLOCK_.header.site_name} 	
    {secondary_links}
    {primary_links}

    {_BLOCK_.header.blocks}
    {_BLOCK_.header.title}
    {submitted}
    {content}

    {_BLOCK_.footer.message}

Obviously, something isn't set up right.  I've seen this happen before, 
but not with my own code.  Does anyone know what's going on here?

Thanks,
Rob