MDL-68674 theme: Have block headers have h3 class="h5".

lib/outputrenderers.php

@@ -4177,9 +4177,10 @@ EOD;
             'data-droptarget' => '1'
         );
         if ($this->page->blocks->region_has_content($displayregion, $this)) {
-            $content = $this->blocks_for_region($displayregion, $fakeblocksonly);
+            $content = html_writer::tag('h2', get_string('blocks'), ['class' => 'sr-only']) .
+                $this->blocks_for_region($displayregion, $fakeblocksonly);
         } else {
-            $content = '';
+            $content = html_writer::tag('h2', get_string('blocks'), ['class' => 'sr-only']);
         }
         return html_writer::tag($tag, $content, $attributes);
     }

lib/templates/block.mustache

@@ -39,7 +39,7 @@
 
         {{! Block header }}
         {{#title}}
-            <h5 id="instance-{{blockinstanceid}}-header" class="card-title d-inline">{{{title}}}</h5>
+            <h3 id="instance-{{blockinstanceid}}-header" class="h5 card-title d-inline">{{{title}}}</h3>
         {{/title}}
 
         {{#hascontrols}}