Dark shadows and overlays in dark mode

What does this MR do and why?

override the mostly-transparent colors in dark mode. Prevents inverting things like modal overlays, and a "glow" effect on shadows.

This is probably too far reaching and some of these things we do want to be lighter colored (dark overlays can potentially be too subtle). But aesthetically this is an improvement over what we currently have, and should really have happened years ago.

Changelog: fixed

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable dark mode
2. Open some modals (new search bar) and sidebars/drawers.

MR acceptance checklist

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