If you want to customize RexMd's module input, you should know what are you doing.
As complex as the input is, as simple is the output.
RexMd's output is just the result of Parsedown's result of the given Markdown input.
By default, RexMd encapsulates the output within a DIV element with the class ytilsRexMd. you can turn this behaviour off in the add-on's settings, or you might change RexMd's output. The default gives you a simple possibility to style the output by simply prefixing your CSS with div.ytilsRexMd.