How can you keep the "theme" of your website and add new dynamic pages?

I'm starting to learn programming, and have pretty much grasped HTML, CSS and JavaScript, now looking to actually pick up a back-end language.

One thing that was never explained is how to update/add content to a webpage without going back to the source code/copying and pasting the html and editing the body.

Also, I would like a brief explanation as to how I can implement a system where users can generate pages. Kickstarter would be a good example, such as how users can make campaigns and have a dedicated page for their campaigns.