feat(theming): dark theme elevation helpers don't match material spec for elevation behavior

#### Feature Description

The material spec for elevation with dark theme (https://material.io/design/color/dark-theme.html#properties), says that elevation should be reflected by an increased lightening of the surface color (by applying more and more opaque white overlays to a base surface color), rather than using drop shadow alone to reflect elevation (since that's not reliable when you have dark surface backgrounds). This request is to have the mat-elevation-z# classes and elevation mixins implement this lightening behavior from the spec, rather than only using drop shadow like it does for light theme.


Start by locating the mat-elevation-z# classes and elevation mixins in the theming implementation, then compare their dark-theme behavior with the linked Material spec. Done means dark-theme elevation reflects increasing surface lightening through the specified overlays rather than relying only on drop shadows; no specific files or tests are named in the issue.

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