Don't apply background contrast behind logout greeter

• Don't apply background contrast behind logout greeter

There's an beautiful full screen blur setup and you don't actually get to see it because the background is effectively black.

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Reason for the change

What’s the blur good for if we don’t actually get to see it? There’s also a 50% backdrop Rectangle anyway which should provide plenty contrast.

Also still readable in front of e.g. a blank text document.

Do we want some backwards compatibility or opt-in to not break other themes that might rely on the contrast entirely?

Screenshots or screen recordings

Before	After

Bugs fixed

Didn’t find any