It’s not possible to inject HTML code with CSS, and so Microthemer doesn’t aim to provide such functionality. In order to allow this, Microthemer would have to become a full blown page builder. This isn’t on the road map.
It is possible to inject HTML via JavaScript, using Microthemer’s JS editor (if you know JavaScript). But in most cases it’s better to add and edit HTML via the normal channels i.e. the WordPress post/page editor or your page builder of choice.