Microthemer should work fine with HTTPS enabled, as long as the frontend and admin area both use HTTPS. And there are no mixed content warnings, which happens if e.g. a single image URL on the forntend uses HTTP instead of HTTPS on a page that is served over HTTPS.
There is no particular setting you need to change for Microthemer to support HTTPS.
Have you encountered any problems with Microthemer since switching to HTTPS, or are you asking in preparation for switching?