Use Admin theme semantic colour variables in Sass variables

Mike Herchelrequested to merge
issue/paragraphs-3619718:admin-theme-dark-mode into 8.x-1.x

Every colour in _variables.scss becomes var(--admin-color-*, ) so the widget follows the core Admin theme's light and dark modes while rendering unchanged in Claro and other themes. Splits the dropdown panel background/shadow and badge text colour into their own variables because they map to different semantic tokens than the hover state.

Co-Authored-By: Claude Fable 5 noreply@anthropic.com Claude-Session: https://claude.ai/code/session_012WvotQwusXDgYyQwuoov29

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