CSS: How to make the column borders of the nth table thicker when the number of columns is variable?

I want to make the circled border thicker, but the number of columns can be between 3 and 6, and I can only use pure CSS (no Javascript, SASS or running CSS through PHP first). HTML is generated by PHP, but changing the PHP code to generate classes or inline styles is a last resort.

I tried using colgroupand col, but while it works , a col:first-child {border-right: 2px solid}thicker col:last-child {border-left: 2px solid}border is used on each column under Percentage.

Is there a way to make the bezel thicker given the above constraints?

As requested by dippas, below is the HTML code in question.

<table class="type1">
  <colgroup>
    <!-- note that "4" could range from 3 to 6 -->
    <col span="1">
    <col span="4">
    <col span="4">
  </colgroup>
  <thead>
    <tr>
      <th scope="row" rowspan="2">S = small M = medium L = large</th>
      <th scope="col" colspan="4">Responses</th>
      <th scope="col" colspan="4">Percentage</th>
    </tr>
    <tr>
      <th scope="col">S</th>
      <th scope="col">M</th>
      <th scope="col">L</th>
      <th scope="col">Tot</th>
      <th scope="col">S</th>
      <th scope="col">M</th>
      <th scope="col">L</th>
      <th scope="col">Tot</th>
    </tr>
  </thead>
  <tbody>
    <!-- following tr structure repeats for each question -->
    <!-- other questions have been removed for brevity -->
    <tr>
      <th scope="row">1. What size Coke do you prefer?</th>
      <td>24</td>
      <td><strong>28</strong></td>
      <td>0</td>
      <td>52</td>
      <td>46</td>
      <td><strong>54</strong></td>
      <td>0</td>
      <td>100</td>
    </tr>
  </tbody>
</table>

Note again that the number of columns is variable. In this case, the possible answers to the question are "Small, Medium, or Large," but it could be "1 to 5, and so on."