The default style sheets show various examples of what you can do, and
they all include CSS that gets applied to the preview pane. You can
modify whichever parts of it you like.
As for how to modify CSS to do what you want, that's up to you. There
are plenty of resources on the internet about how to use CSS, but you
will need to be creative to apply them to HTML generated from MMD, since
it doesn't (normally) have different classes for different instances of
element (e.g. all `<p>` tags are the same). Alternatively you can use
lots of raw HTML in your MMD document, but that defeats the purpose of
using MMD in the first place.