Cover block: unlock ability to set specific percentage on overlay #82083

What problem does this address?

Currently it seems impossible to set a specific percentage on the overlay in a Cover Block. It snaps to the nearest 10 value and there doesn't seem to be a way to override this.

For example, it's impossible to set the overlay to 93%. It snaps to 90%.

This can be crucial for making text accessible by making the background dark enough.

Also, I'd expect to be able to set percentages on other controls as well. I don't see a good reason why this one is snapping to the nearest 10.