Elevation.

On a paper-white canvas, depth is cast — the higher a surface sits, the softer and longer its shadow. Dialogs and popovers use shadow-3. Toolbars stay at shadow-1. Cards never use shadow alone — a hairline border does the work.

In the dark, cast shadows disappear — there's no light to cast them. So we invert the model: each elevation step lightens the surface. The dialog floats by being brighter than the drawer behind it, not by sitting on a shadow that doesn't exist.