Theme: Fix contrast in nested story

[tyxla]

What?

Fixes a warning in the Themes nested story.

Why?

Just fixing a warning in the story.

Found while working on #67574.

How?

We're using an accent color that achieves sufficient contrast against the background color.

Testing Instructions

• Open the nested Theme story: http://localhost:50240/?path=/story/components-theme--nested
• Verify that there is no contrast warning in the console.

Testing Instructions for Keyboard

Same

Screenshots or screencast

Error before this PR:

[Mamaduka]

LGTM ✅

P.S. We need to fix the same warning in the Site Editor.

Screenshot

[github-actions]

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[github-actions]

Flaky tests detected in 95c83e9.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/12196446046
📝 Reported issues:

• [Flaky Test] submenu click on the arrow interactions #61884 in /test/e2e/specs/editor/blocks/navigation-frontend-interactivity.spec.js

[tyxla]

P.S. We need to fix the same warning in the Site Editor.

Hmm, isn't that related to the active theme? I'd expect the color needs to be changed there and it's not really a core issue.