How to do it...

To create a runtime UI Toolkit interface, follow these steps:

1. Create a new, empty 3D project.
2. Add the UI Toolkit and UI Builder packages to the project using Package Manager (see the Getting ready section).
3. Open the UI Builder panel and go to Window | UI Toolkit | UI Builder.
4. Rearrange the panels (personally, I follow the Unite Copenhagen demo layout):

• LEFT HALF: UI Builder 
• RIGHT HALF: Top: Scene + Game + Inspector + Console, with Game showing
• RIGHT HALF: Bottom:  Hierarchy + Project, with Hierarchy showing:

5. Viewport is the center panel of the UI Builder. From its File menu, choose Save and save the current empty new document as ui-1. You'll see a ui-1 UXML document appear in the Project window – it will have a child USS named inline style...