添加 .table-bordered 类为表格和其中的每个单元格增加边框。

<table class="table table-bordered">
      <thead>
        <tr>
          <th>#</th>
          <th>First Name</th>
          <th>Last Name</th>
          <th>Username</th>
        </tr>
      </thead>
      <tbody>
        <tr>
          <th scope="row">1</th>
          <td>Mark</td>
          <td>Otto</td>
          <td>@mdo</td>
        </tr>
        <tr>
          <th scope="row">2</th>
          <td>Jacob</td>
          <td>Thornton</td>
          <td>@fat</td>
        </tr>
        <tr>
          <th scope="row">3</th>
          <td>Larry</td>
          <td>the Bird</td>
          <td>@twitter</td>
        </tr>
      </tbody>
    </table>