It turns out that the ThemeWoot Emulate theme registers and enqueues a common.css file with the key of common in it’s themewoot.php (which is being included in functions.php).
This key is apparently conflicting with a wp-admin script being registered with the same common key, and so, instead of including Emulate’s common.css, it’s enqueuing and injecting the wp-admin common.min.css.
I edited the themewoot.php and namespaced the common key with a an emulate- prefix:
From:
wp_register_style('common', $this->theme_url(). '/css/common.css', false, TWOOT_VERSION, 'all');
wp_enqueue_style('common');
To:
wp_register_style('emulate-common', $this->theme_url(). '/css/common.css', false, TWOOT_VERSION, 'all');
wp_enqueue_style('emulate-common');
And that got things back in working order.
ThemeWoot doesn’t appear to be maintaining the Emulate theme anymore. In reality they should patch this and provide an update.
<rant>
WordPress should seriously consider namespacing their core styles and scripts so that updates don’t affect themes like this. A simple wp- would have gone a long way here and saved me at least a few hours. Theme creators should also namespace all of their assets too so that they play nice with everything else.
</rant>