This can be your "sales page".
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 E.M.M.A. Default (views/master.html)
Of course, you can always just edit views/master.html instead of using the template system, but be careful not to mess it.