In previous versions of SharePoint, a lot of the look and feel of a site was driven by a combination of editing master pages, custom CSS, and custom scripting. Those configurations and customizations may have worked in an environment where the majority of the devices that were connecting were managed desktops, but in the modern infrastructure, users are accessing content from a variety of devices, including personal computers and mobile devices. In addition to running into potential display issues, introducing custom code can also present security risks. With modern SharePoint, we want to focus on customizing the built-in responsive designs through the use of branding, navigation, and page layout options with web parts as much as possible.