Add dark theme CSS selector for navbar and dropdown

[pinggi]

Description

Adds [data-bs-theme="dark"] selector to navbar and dropdown-menu, so it's possible to set dark color mode for the components by setting data-bs-theme="dark" attribute to html element (or any parent element of those components).

Motivation & Context

The dark theme set on html element wasn't applied to navbars and dropdowns. See the #41310.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-{your_pr_number}--twbs-bootstrap.netlify.app/

Related issues

#41310