To change a page template layout, you can copy the file to your theme folder. For example, to edit the header, you would copy gatsby-theme-publisher/src/components/Header.js into your-theme/src/components/Header.js and edit the file there. Gatsby will automatically use your header file instead of the default.
This theme uses Chakra UI, which gives you a lot of easy to use components. You can use any of these components in your theme templates.