Color mode: Offer an alternative `data-bs-theme="body"`

[louismaximepiton]

Description

:warning: Make sure the example is removed before merge.

Add a special "theme" that only replicate the main theme of the page.

❓ If this theme is accepted, we might introduce data-bs-theme="inverted" that inverts the global theme. Here is an example of how it could be implemented atm: https://codepen.io/louismaximepiton/pen/vYbEvLO.

Motivation & Context

Get rid of dark variants. Quality of life for end-users. New feature that adds functionality to help people find a way to have the previous behavior (v5.2.3) (without JS)

Type of changes

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

Checklist

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

Live previews

On this test page that has to be removed, the rendering of every kind of use of navbar and its associated dropdown.

• https://deploy-preview-39188--twbs-bootstrap.netlify.app/docs/5.3/examples/test
• https://deploy-preview-39188--twbs-bootstrap.netlify.app/docs/5.3/customize/color-modes#always-go-back-to-main-theme

Related issues

Closes #38973.