dunst: set highlight color to match frame color

[trueNAHO]

Testing this PR with https://github.com/nix-community/stylix/pull/2021, results in the following changes:

Before	After

This PR is a follow-up to https://github.com/nix-community/stylix/pull/2011.

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• [X] Changes have been tested in testbeds
• [X] Each commit in this PR is suitable for backport to the current stable branch