1.To add widgets, go to WordPress Menu > Appearance > Arcade Themes Setting > Widgets
2. It will appear the customize setting of widgets. At you left hand side, it is an available widgets. Then, at your right hand side, it is a visual representation of the widget area supported by your currents pages. There are 3 widgets areas, sidebar, which is on your right hand side , below header and footer. Click at manage in customizer.
3. You will see on your left side the list of widgets that you can customize.
4. For sidebar, you can customize what must have in recent posts, recent comments, archives, categories and meta. Besides that, you also can add a widget.
5. For below header, you can add anything that you want on below header.
6. For footer, you can add anything that you want at the footer.