It recently came to my attention that there was an error version 7’s sample PHP code for manually loading the CSS, when deactivating Microthemer. This has been fixed in version 188.8.131.52. So please update MT and then copy the new code to your child theme’s functions.php file, replacing the existing code.
Get the sample code inside Microthemer via:
Settings > General > Preferences > Inactive
The error was also present in the simple plugin, which can be downloaded from the Inactive tab too.
Apologies for the hassle. Updating the code is less convenient than updating MT.