Bootstrap on Twitter: "v5.2.0 brings an updated border-radius to Bootstrap. Subtle and refreshing. 馃槍 Also includes new global CSS variables like `--bs- border-radius` and two new classes, `.rounded-4` and `.rounded-5`. Peep the PR
css - How to use border with Bootstrap - Stack Overflow
Fix Border styling with Bootstrap library hard overrides over borders [#3230802] | Drupal.org