bootstrap4.css
.btn-outline-dark {
  color: #343a40;
  border-color: #343a40;
}
bootstrap4.css
.btn-outline-dark:hover {
  color: #fff;
  background-color: #343a40;
  border-color: #343a40;
}
bootstrap4.css
.btn-outline-dark:focus, .btn-outline-dark.focus {
  box-shadow: 0 0 0 0.2rem rgba(52, 58, 64, 0.5);
}
bootstrap4.css
.btn-outline-dark.disabled, .btn-outline-dark:disabled {
  color: #343a40;
  background-color: transparent;
}
bootstrap4.css
.btn-outline-dark:not(:disabled):not(.disabled):active, .btn-outline-dark:not(:disabled):not(.disabled).active,
.show > .btn-outline-dark.dropdown-toggle {
  color: #fff;
  background-color: #343a40;
  border-color: #343a40;
}
bootstrap4.css
.btn-outline-dark:not(:disabled):not(.disabled):active:focus, .btn-outline-dark:not(:disabled):not(.disabled).active:focus,
.show > .btn-outline-dark.dropdown-toggle:focus {
  box-shadow: 0 0 0 0.2rem rgba(52, 58, 64, 0.5);
}
■ボタン:button:コンポーネント:アウトラインボタンを作成する

  <button class="btn btn-outline-primary">outline-primary</button>
  <button class="btn btn-outline-secondary">outline-secondary</button>
  <button class="btn btn-outline-success">outline-success</button>
  <button class="btn btn-outline-danger">outline-danger</button>
  <button class="btn btn-outline-warning">outline-warning</button>
  <button class="btn btn-outline-info">outline-info</button>
  <button class="btn btn-outline-light">outline-light</button>
  <button class="btn btn-outline-dark">outline-dark</button>