Theme for someone else – page creation

So I’m creating a theme for someone, and I’m wondering what the usual way is to go about creating pages. I suspect they will want to create new pages themselves at some point, so they should have the ability to do so. I’m used to creating static HTML/CSS sites so I don’t know what I need to do in regards to this? I have an index.php file which presumably will be used for any page they create. I have styles for contact forms. How do I make it so it will add one of my buttons when they make a new page? I have them set up as a class and they currently sit in a navigation div, but obviously the buttons I currently have don’t link to anything.

Hope this makes sense, and thank you for any help. (This is the right forum to post in?)