mdb-ui-kit/scss/_tables.scss

// BS4 has not yet converted the following to variables - keep an eye on it and move to variables when possible.
.thead-inverse {
  th {
    //color: $bmd-inverse; // #fff;
    //background-color: $gray-dark;
  }
}
.thead-default {
  th {
    //color: $gray;
    //background-color: $bmd-inverse-lighter; //$gray-lighter;
  }
}

.table-inverse {
  color: $bmd-inverse-light; //$gray-lighter;
  //background-color: $gray-dark;

  //th,
  //td,
  //thead th {
  //  border-color: $gray;
  //}
}

.table {
  thead th {
    font-size: $bmd-table-header-font-size;
    font-weight: 500;
    color: $gray;
    border-top-width: 0;
    border-bottom-width: $table-border-width;
  }
}

thead.thead-inverse,
// needs specificity
.table-inverse thead {
  th {
    color: $bmd-inverse-lighter;
  }
}

.table-inverse {
  th,
  td,
  thead th {
    border-color: $bmd-table-border-color-inverse;
  }
}