When working with the default styles of Bootstrap 5.x – I ran into seeing a blue outline around my nav items in the nav bar.

Initial State

Without the Outline

Another Option

In order to get rid of it (in favor of either no outline or something more custom) you will simply need to add in this line of CSS:

nav .dropdown a.show {outline:none;box-shadow:none;}

That will then remove that blue outline when the dropdown is open. It then also allows you to add a background or any other style ( to the a.show class) to the dropdown which will likely fit int a custom design more.