One door opens, another closes.
Since I upgraded to the new version of Microthemer, my CSS targeting no longer works.
I double click on an item on the page, and the css targeting box is blank.
No matter what I did, I could not get Microthemer CSS targeting to work. I disabled all my plugins, tried it on different computers with Chrome, Firefox, Edge and IE10.
So at this point I just disabled it. I used your code to load active-styles.css in my child-theme functions.php. file.
It must be something with this particular theme. I was able to make most of my changes with Microthemer before it crapped out, so all is not lost.