CODE EXAMPLE FOR CSS

wrapped header aura datatable

.THIS .wrapped-header-datatable .slds-table .slds-th__action .slds-truncate{
      max-width: 100%;
      overflow: hidden;
      text-overflow: unset;
      white-space: pre-line;
      /*  uncomment this if you want break the words further */
      /* word-wrap: break-word;*/

  }

 .THIS .wrapped-header-datatable .slds-table .slds-th__action {
      height: 5rem;
      border-bottom: 1px solid rgb(206 206 206);
  }

 .THIS .wrapped-header-datatable .slds-table .slds-line-height_reset{
      height: 3rem;
  }