wagtail/client/scss/components/_button-select.scss

.button-select {
  &__option {
    display: block;
    width: 100%;
    margin-bottom: 10px;

    background-color: #fff;
    border-color: $color-teal;
    color: #262626;

    &--selected {
      background-color: $color-teal;
      color: #fff;
    }
  }
}

.button-select .button-select__option {
  /* override default margin from horizontally-aligned buttons */
  margin-inline-start: 0;
}