feat: Add background colour and full-height prop to Screen

[VincentSmedinga]

The initial value for background-color is transparent. I can imagine either dark mode or an OS or browser theme setting a background colour, but I’d rather be sure it’s white. We can adapt for dark mode later.

Another reason to define it explicitly is that even though a white background may come by default sometimes, it is an explicit part of our design that other components rely on.

We need the 100vh height for applications, e.g. to accommodate the side menu. We also need it to ensure the white background spans from top to bottom.