bug(mat-expansion): expose box-shadow as css variable

[RobinKamps]

Is this a regression?

• Yes, this behavior used to work in the previous version

The previous version in which this bug was not present was

No response

Description

Please add a css variable to set the box-shadow for easy access / theming:

.mat-expansion-panel:not([class*=mat-elevation-z]) {
    box-shadow: 0px 3px 1px -2px rgba(0, 0, 0, 0.2), 0px 2px 2px 0px rgba(0, 0, 0, 0.14), 0px 1px 5px 0px rgba(0, 0, 0, 0.12);
}

Reproduction

StackBlitz link:
Steps to reproduce:
1.
2.

Expected Behavior

drop shadow of panel could be set via a css variable

Actual Behavior

drop shadow of panel can not set via a css variable

Environment

• Angular: latest / 20x
• CDK/Material: 20.0.2
• Browser(s):
• Operating System (e.g. Windows, macOS, Ubuntu):

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