bug: Alert wrapper in dark mode is still using light background

[brandyscarney]

Bug Report

Ionic version:

[x] 4.x

Current behavior:
Alert overlay background uses --ion-overlay-background-color for the background but does not fallback to the step color in Material Design only.

Expected behavior:

    --background: var(--ion-overlay-background-color,#fff);

This should behave like iOS, falling back to a step color.

Steps to reproduce:

Run the conference app in dark mode.

Related code:

insert short code snippets here

Other information:

Ionic info:

insert the output from ionic info here

[ionitron-bot]

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