Add Dark Mode support for MR widget empty state icon

What does this MR do and why?

Closes: #330195 (closed)

The colors I used are a mixture of colors taken from the legacy Illustration Library and manual contrast-matching.

Screenshots or screen recordings

	Before	After
White Mode (no visible change)
Dark Mode

Icon in a context

How to set up and validate locally

1. Create an empty merge request with no changes
2. Switch to Dark Mode via preferences

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.