Fix index link text color for current sphinx-gallery version

[guberti]

After the sphinx-gallery version was updated by apache/tvm#10921, it became difficult to see the text color of links in the gallery index page:

This occurred because of a change in the way the text was displayed in HTML. Previously, the text elements had the following structure:

<div class="sphx-glr-thumbcontainer" tooltip="This tutorial is an introduction to working with microTVM and a TFLite model with Relay.">
  <div class="figure align-default" id="id3">
    <img alt="../../_images/sphx_glr_micro_tflite_thumb.png" src="../../_images/sphx_glr_micro_tflite_thumb.png">
    <p class="caption">
      <span class="caption-text">
        <a class="reference internal" href="micro_tflite.html#sphx-glr-how-to-work-with-microtvm-micro-tflite-py">
          <span class="std std-ref">microTVM with TFLite Models</span>
        </a>
      </span>
      <a class="headerlink" href="#id3" title="Permalink to this image">¶</a>
    </p>
  </div>
</div>

However, in current versions this is instead, where the <a> tag is stylized to make everything in the outer <div> a link:

<div class="sphx-glr-thumbcontainer" tooltip="This tutorial is an introduction to working with microTVM and a TFLite model with Relay.">
  <img alt="microTVM with TFLite Models" src="../../_images/sphx_glr_micro_tflite_thumb.png">
  <p>
    <a class="reference internal" href="micro_tflite.html#sphx-glr-how-to-work-with-microtvm-micro-tflite-py">
      <span class="std std-ref">microTVM with TFLite Models</span>
    </a>
  </p>
  <div class="sphx-glr-thumbnail-title">microTVM with TFLite Models</div>
</div>

Because of this change, our previous CSS rule (.rst-content .sphx-glr-thumbcontainer a.internal) is unable to locate the relevant text, and is unable to make it white. This PR changes that selector to fix the bug and make the text display correctly again.