Accessibility: Add aria-label to clearly describe the dark mode button (#2426)

pep_sphinx_extensions/pep_theme/templates/page.html

@@ -25,7 +25,7 @@
                 <li><a href="{{ pathto("pep-0000") }}">PEP Index</a> &raquo; </li>
                 <li>{{ title }}</li>
             </ul>
-            <button onClick="toggleColourScheme()"></button>
+            <button aria-label="Toggle dark mode" onClick="toggleColourScheme()"></button>
         </header>
         <article>
             {{ body }}