Docs: enhance callouts links on dark mode

Following up https://github.com/twbs/bootstrap/pull/37759/files.

This PR suggests to handle different colors for the links within the callouts based on light/dark mode to avoid the following use cases.

Before

Now

Warning callout

Info callout

• https://deploy-preview-37761--twbs-bootstrap.netlify.app/docs/5.3/components/carousel/