There used to be an option in the Themeover plugin’s admin menu to edit the CSS.
Has it moved or been taken away?
All I seem to be able to do is view the code, via the vertical menu, which I can see little point in doing if I can’t amend it.
It may have moved since you last used Microthemer but the custom code feature is still there. You can access it from the custom code icon in the top right of the interface. The view and edit custom code options will be next to each other in an upcoming release (I appreciate that current separation isn’t intuitive).
Please let me know if you have any further questions.