[MatButtonToggleGroup] cannot unset elevation

[merlosy]

Bug, feature request, or proposal:

The class "mat-elevation-z0" doesn't change elevation style

What is the expected behavior?

I should be able to unset the elevation with the class "mat-elevation-z0"

What is the current behavior?

It does nothing

What are the steps to reproduce?

StackBlitz: https://stackblitz.com/edit/angular-pqeknw?file=app%2Fbutton-toggle-exclusive-example.html

What is the use-case or motivation for changing an existing behavior?

It seems to be the proper to unset it.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular: 5
Angular Material: 5
Windows 8.1

Is there anything else we should know?

CSS rules seem to be in the wrong order.
Also, I believe the text (options) is a bit too bright, and the is not recommendation for toggle buttons with text on https://material.io/guidelines/components/buttons.html#buttons-toggle-buttons

[crisbeto]

Should be possible once #7909 gets in.

[crisbeto]

Should've been resolved some time ago by #14722.

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