site_theme_paths page forbidden issues

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I have a mobile theme setup using the free block available in the marketplace. However, even though I have a 'page_forbidden.php' file in my mobile theme directory, it always shows the non-mobile theme forbidden page instead.

I even added the following to my 'site_theme_paths.php' file but no change. If I add the exact theme path for mobile it shows the correct forbidden page but for some reason using getThemeHandle doesn't give the same results. Any thoughts?

$v = View::getInstance();
$v->setThemeByPath('/login',  $v->getThemeHandle());
$v->setThemeByPath('/page_forbidden', $v->getThemeHandle());
$v->setThemeByPath('/page_not_found', $v->getThemeHandle());