It’s a bit fiddly. I would recommend pasting the code below via Microthemer’s full code editor (View > Code editor). You need to use the :placeholder pseudo selector. Unfortunately this hasn’t standardised yet. And so in order to get this working across many browsers, you will need to repeat your styles for each vendor-specific selector:
If you know SASS, you could enable support for it via the MT preferences and then write a mixin to save some repetition. But if you don’t know SASS, a little repetition is probably the simplest approach.