1. html

Use the class 'btn' and 'btn-rounded'

                  <button type="button" class="btn btn-primary btn-rounded">Primary</button>
                  <button type="button" class="btn btn-secondary btn-rounded">Secondary</button>
                  <button type="button" class="btn btn-success btn-rounded">Success</button>
                  <button type="button" class="btn btn-danger btn-rounded">Danger</button>
                  <button type="button" class="btn btn-warning btn-rounded">Warning</button>
                  <button type="button" class="btn btn-info btn-rounded">Info</button>
                  <button type="button" class="btn btn-light btn-rounded">Light</button>
                  <button type="button" class="btn btn-dark btn-rounded">Dark</button>