Andrew Roberts has shared a tutorial and boilerplate for using the Materialize CSS framework in the Google Sheets sidebar. Materialize is based on Material Design and the framework is lightweight and easy to use. This framework will also work for other G Suite editor sidebars (Docs, Sheets, Forms, Slides). If you plan to use Materialize in your Editor Add-on Google ask that the design adheres to the UI style guide.
Source: GSheet Sidebar with Materialize CSS & jQuery – Andrew Roberts
Member of Google Developers Experts Program for Google Workspace (Google Apps Script) and interested in supporting Google Workspace Devs.