Once on the Email invitations page from the dashboard there will be a custom design / Preview option. Choosing this will allow you to edit the content of the invitation on the left hand side as well as change your theme at the top of the preview image.
All content is completely customisable and once saved will show on the preview image. You can use the drop down list of guests on the left hand side to view exactly how the invitation will be sent to that specific guest.
If you leave then return to your email invitations a checklist will show as a reminder of things you might like to include on your invites. To skip this, simply chose the 'not needed' option for all listing. While the checklist will not show again, You can still access the tools mentioned on the list at anytime in the corresponding blocks in your dashboard.