Dark theme fixes for navbar and dropdown (Fixes #39984)

[karlshea]

Description

• Allow data-bs-theme="dark" on parent element of .navbar to trigger .navbar-dark variables as described in the documentation.
• Allow data-bs-theme="dark" on parent or current element of .dropdown-menu to trigger .dropdown-menu-dark variables as described in the documentation.

Motivation & Context

Closes #39984

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-39989--twbs-bootstrap.netlify.app/docs/5.3/components/dropdowns/
• https://deploy-preview-39989--twbs-bootstrap.netlify.app/docs/5.3/components/navbar/

Related issues

#39984, #39291

[julien-deramond]

Thanks for submitting a PR @karlshea

I haven't looked at the code and the modifications that you suggest yet. However, I can see in the deployed version a rendering issue in the dark mode version of https://deploy-preview-39989--twbs-bootstrap.netlify.app/docs/5.3/components/navbar/#color-schemes, where the last navbar should render black texts instead of white:

[karlshea]

@julien-deramond I'm not sure how the last navbar could be fixed if the background color is set with a style tag, there doesn't seem to be anything to work off of there?

In 5.3 without this PR's change if I remove bg-primary from the second navbar and manually set the background color with a style the text also renders white in dark mode.

[julien-deramond]

I've checked rapidly, but there is a forced data-bs-theme="light" (that should be displayed in the markup of the documentation, by the way). In this case, the color should be black since the light mode is forced.

I'll need to find the time to check all the existing PRs and issues at the same time regarding the color modes, because this could be also fixed automatically by #39295 if the color: is set by default (not the case right now).

[karlshea]

Yeah I'm not really sure what to do here. Maybe CSS specificity is winning?