.ms-parent.form-control {
  padding: 0;

  .ms-choice {
    height: 100%;
    border: 0;

    > span {
      top: 50%;
      transform: translateY(-50%);
    }
  }

  &.form-control-sm {
    .ms-drop input[type=radio],
    .ms-drop input[type=checkbox] {
      margin-top: 0.4rem;
    }
  }

  &.form-control-lg {
    .ms-drop input[type=radio],
    .ms-drop input[type=checkbox] {
      margin-top: 0.5rem;
    }
  }

  &.ms-dropdown {
    .ms-drop ul > li {

      &.group.hide-radio label.optgroup {
        margin-bottom: 0;
        font-size: .875rem;
        color: #6c757d;
        white-space: nowrap;
        opacity: 1;
      }

      &.hide-radio.option-level-1 label {
        padding-left: 8px;
      }
    }

    &.ms-dropdown-divider {
      .ms-drop ul > li {

        &.group.hide-radio label.optgroup {
          border-top: 1px solid #e9ecef;
        }

        &.group:first-child label.optgroup {
          border-top: none;
        }
      }
    }
  }
}

.ms-parent.form-control {
  padding: 0;

  .ms-choice {
    height: 100%;
    border: 0;

    > span {
      top: 50%;
      transform: translateY(-50%);
    }
  }

  &.form-control-sm {
    .ms-drop input[type=radio],
    .ms-drop input[type=checkbox] {
      margin-top: 0.4rem;
    }
  }

  &.form-control-lg {
    .ms-drop input[type=radio],
    .ms-drop input[type=checkbox] {
      margin-top: 0.5rem;
    }
  }

  &.ms-dropdown {
    .ms-drop ul > li {

      &.group.hide-radio label.optgroup {
        margin-bottom: 0;
        font-size: 87.5%;
        color: #6c757d;
        white-space: nowrap;
        opacity: 1;
      }

      &.hide-radio.option-level-1 label {
        padding-left: 8px;
      }
    }

    &.ms-dropdown-divider {
      .ms-drop ul > li {

        &.group.hide-radio label.optgroup {
          border-top: 1px solid #e9ecef;
        }

        &.group:first-child label.optgroup {
          border-top: none;
        }
      }
    }
  }
}

Update font-size to use percent instead
Update font-size to use percent instead
