Define theme regions so only specified regions are visible in Appearance settings
>>> [!note] Migrated issue
<!-- Drupal.org comment -->
<!-- Migrated from issue #3561866. -->
Reported by: [balintbrews](https://www.drupal.org/user/613760)
Related to !139
>>>
<h3>Problem/Motivation</h3>
<p>Mercury defines no theme regions, so the defaults from <a href="https://git.drupalcode.org/project/drupal/-/blob/11.2.9/core/lib/Drupal/Core/Extension/ThemeExtensionList.php?ref_type=tags#L23"><code>\Drupal\Core\Extension\ThemeExtensionList::$defaults</code></a> are used:</p>
<ul>
<li>Left sidebar</li>
<li>Right sidebar</li>
<li>Content</li>
<li>Header</li>
<li>Primary menu</li>
<li>Secondary menu</li>
<li>Footer</li>
<li>Highlighted</li>
<li>Help</li>
<li>Page top</li>
<li>Page bottom</li>
<li>Breadcrumb</li>
</ul>
<p>These defaults are no longer appropriate in the context of Canvas, and the Mercury components are not designed to make use of anything outside of Header, Content, and Footer — which are also the only regions enabled for Canvas to manage in Byte.</p>
<h3>Proposed resolution</h3>
<p>Define the following regions in Mercury:</p>
<ul>
<li>Content</li>
<li>Header</li>
<li>Footer</li>
</ul>
<h3>User interface changes</h3>
<p><strong>Theme settings — before:</strong></p>
<p><img src="https://www.drupal.org/files/issues/2025-12-07/mercury-theme-regions-defaults.jpg" alt="Mercury theme settings with default regions"></p>
<p><strong>Theme settings — after:</strong></p>
<p><img src="https://www.drupal.org/files/issues/2025-12-07/mercury-theme-regions-new.jpg" alt="Mercury theme settings with proposed regions"></p>
issue