Formatting changes when adding a new duplicate page

[htmlnewb]

I'm currently building a very basic website for a school project. There has to be 5 pages including the home page. The home page has been set up how I want it and the teacher basically just wants us to use the same css/html5 coding to create the other 5 pages. So I have copied and pasted, changed the link rel however the css formatting completely changes when clicking on the new duplicate pages I set up.