Why does this css not center the submit button on this form?
<p>I want to center this submit button, using CSS, and have researched this in many other online posts, but have not yet found a solution, which seems odd for something that should be so simple.</p>

<p><div class="snippet" data-lang="js" data-hide="false">
<div class="snippet-code">
<pre class="snippet-code-css lang-css prettyprint-override"><code>td.class &gt; div {
  width: 100%;
  height: 100%;
  overflow: hidden;
  height: 20px;
}
#buttonstyle {
  border: 2px solid #777777;
  background: #019966;
  color: black;
  font: bold 18px'Trebuchet MS';
  padding: 4px;
  cursor: pointer;
  width: 150px;
  border-radius: 12px;
  display: inline-block;
  margin: 0px auto;
}
.buttonHolder {
  text-align: center;
}
.heading {
  font-family: Verdana, Arial, Helvetica, sans-serif;
  font-size: 16px;
  font-weight: bold;
  text-align: center;
  color: #006633;
}
input {
  font-family: Verdana, Arial, Helvetica, sans-serif;
  font-size: 10px;
  font-weight: normal;
}
input[type=text] {
  font-family: Verdana, Arial, Helvetica, sans-serif;
  font-size: 12px;
  font-weight: normal;
}
input[type=date] {
  font-family: Verdana, Arial, Helvetica, sans-serif;
  font-size: 12px;
  font-weight: normal;
}
ul {
  list-style: none;
  text-align: left;
}
input {
  width: 20px;
  position: relative;
  left: 150px;
  vertical-align: middle;
}
label {
  width: 200px;
  position: relative;
  left: -20px;
  display: inline-block;
  vertical-align: middle;
}</code></pre>
<pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;form name="InstructorForm" action="../instructorDB_protected/instructorDB_controller.php" method="post" enctype="multipart/form-data"&gt;
  &lt;div id='content'&gt;
    &lt;center&gt;
      &lt;span class="heading"&gt;Colorado Mountain Club - Boulder Group Instructor Database&lt;br&gt;Instructor Administrator Export Form&lt;/span&gt;
      &lt;/p&gt;
      &lt;p&gt;
        Select which schools to export to a file to download.
        &lt;br&gt;The file will contain a list of instructor names, emails and phone numbers:
        &lt;p&gt;
          &lt;div id="yourdiv" style="display: inline-block;"&gt;
            &lt;ul&gt;
              &lt;li&gt;
                &lt;input type="checkbox" name="avi_instructor"&gt;
                &lt;label&gt;Avalanche:&lt;/label&gt;
              &lt;/li&gt;
              &lt;li&gt;
                &lt;input type="checkbox" name="hiking_instructor"&gt;
                &lt;label&gt;Hiking:&lt;/label&gt;
              &lt;/li&gt;
              &lt;li&gt;
                &lt;input type="checkbox" name="ice_instructor"&gt;
                &lt;label&gt;Ice:&lt;/label&gt;
              &lt;/li&gt;
              &lt;li&gt;
                &lt;input type="checkbox" name="rock_instructor_trad"&gt;
                &lt;label&gt;Rock, Trad:&lt;/label&gt;
              &lt;/li&gt;
              &lt;li&gt;
                &lt;input type="checkbox" name="rock_instructor_sport"&gt;
                &lt;label&gt;Rock, Sport:&lt;/label&gt;
              &lt;/li&gt;
              &lt;li&gt;
                &lt;input type="checkbox" name="ski_instructor"&gt;
                &lt;label&gt;Ski:&lt;/label&gt;
              &lt;/li&gt;
              &lt;li&gt;
                &lt;input type="checkbox" name="snow_instructor"&gt;
                &lt;label&gt;Snow:&lt;/label&gt;
              &lt;/li&gt;
            &lt;/ul&gt;
          &lt;/div&gt;
          &lt;br&gt;
          &lt;div class="buttonHolder"&gt;
            &lt;input id=buttonstyle type="submit" name="submitAdminExport" value="Export"&gt;
          &lt;/div&gt;
          &lt;br&gt;
          &lt;a href="../instructorDB_protected/instructorDB_top.php" target="_self"&gt;Return to top&lt;/a&gt;
    &lt;/center&gt;
  &lt;/div&gt;
&lt;/form&gt;</code></pre>
</div>
</div>
</p>

<p>I have experimented with removing three other CSS files that get loaded, which did not make any difference: the submit button refuses to center, and aligns itself along the right margin of the checkboxes, in the  list above it. Any help much appreciated!</p>