select2/src/scss/_single.scss

.select2-container {
  .selection .single-select {
    background-color: #eee;
    border: 1px solid #aaa;
    border-radius: 4px;
    box-sizing: border-box;

    cursor: pointer;
    display: block;

    height: 28px;

    .rendered-selection {
      color: #444;
      display: block;
      line-height: 28px;
      overflow: hidden;
      text-overflow: ellipsis;
    }
  }

  &.open {
    .selection .single-select {
      border-bottom: none;
      border-bottom-left-radius: 0;
      border-bottom-right-radius: 0;
    }
  }
}