Update dark toasts

[simurai]

This is a follow-up to https://github.com/github/primer/issues/57#issuecomment-804044793 and updates the dark toast colors:

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[simurai]

@auareyou The icon-border is not defined in the spec here https://github.com/github/primer/issues/57#issuecomment-804044793, so I just used $gray-3 (one level higher than icon-bg).