bootstrap4.css
.btn-dark {
  color: #fff;
  background-color: #343a40;
  border-color: #343a40;
}
bootstrap4.css
.btn-dark:hover {
  color: #fff;
  background-color: #23272b;
  border-color: #1d2124;
}
bootstrap4.css
.btn-dark:focus, .btn-dark.focus {
  box-shadow: 0 0 0 0.2rem rgba(82, 88, 93, 0.5);
}
bootstrap4.css
.btn-dark.disabled, .btn-dark:disabled {
  color: #fff;
  background-color: #343a40;
  border-color: #343a40;
}
bootstrap4.css
.btn-dark:not(:disabled):not(.disabled):active, .btn-dark:not(:disabled):not(.disabled).active,
.show > .btn-dark.dropdown-toggle {
  color: #fff;
  background-color: #1d2124;
  border-color: #171a1d;
}
bootstrap4.css
.btn-dark:not(:disabled):not(.disabled):active:focus, .btn-dark:not(:disabled):not(.disabled).active:focus,
.show > .btn-dark.dropdown-toggle:focus {
  box-shadow: 0 0 0 0.2rem rgba(82, 88, 93, 0.5);
}
■ボタン:button:コンポーネント:ボタンの色

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