Feature: Updated the fallback color for Acrylic

[heftymouse]

This PR changes the Acrylic fallback colour on dark mode to #202020 which is a much more pleasing colour also used by Mica.

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[yaira2]

It also changes the sidebar and top bar border to use the divider stroke brush making it more consistent.

Can you separate this change? It needs to be discussed with design and I don't want to hold back the other improvements.