ayman1

Turns out that, since 2.5.0, there is a function called switch_theme()! It’s in wp-includes/theme.php:

[Code moderated as per the Forum Rules. Please use the pastebin]

So, it looks like the main thing I was missing was the do_action() at the end. So, the code should be:

$theme = get_theme_of_today();
switch_themes($theme['Template'], $theme['Stylesheet']);
function get_theme_of_today() {
   $day = (int) date('z');
   $themes = get_themes();
   $i = $day % count($themes);
   return $themes[$i];
}