E.M.M.A. Comes with this default template which you can easy modify to turn into your sales page. Just edit the file views/main.html.
You do not have to modify this page - you can as well just skip it and put links on your site or blog directly to the sign-up and login pages of your membership site.
If you want to edit the overal look of the membership site (including the logged member's area) - CSS, layout etc., it's recommended to use the simple and easy template mechanism provided in the administration. The currently active layout is Basic responsive
Of course, you can always just edit views/master.html instead of using the template system, but be careful not to mess it.