https://weekly.ci.jenkins.io/design-library/Banner/ describes how to create banners. This makes use of the css classes "alert-warning", "alert-info", "alert-danger" and "alert-success".

At least "alert-warning" and "alert-info"  and "alert-danger" have a conflict when a page with such a banner also includes parts that lead to loading of bootstrap5 styles (e.g. warnings-ng)

E.g. the effect is that the banner gets a border in the same color as the text.,

This was especially visible in older Jenkins version before https://github.com/jenkinsci/jenkins/pull/9115 was merged, but also affects current Jenkins version:

Expected:

Actual:

Originally reported as https://github.com/jenkinsci/customizable-header-plugin/issues/99