diff --git a/core/modules/help_topics/help_topics/core.appearance.html.twig b/core/modules/help_topics/help_topics/core.appearance.html.twig
index 78bfcf9be6..cb4711f4db 100644
--- a/core/modules/help_topics/help_topics/core.appearance.html.twig
+++ b/core/modules/help_topics/help_topics/core.appearance.html.twig
@@ -7,6 +7,8 @@ related:
 {% set entities = render_var(url('help.help_topic', {'id': 'core.content_structure'})) %}
 <h2>{% trans %}What is a theme?{% endtrans %}</h2>
 <p>{% trans %}A <em>theme</em> is a set of files that define the visual look and feel of your site. The core software and modules that run on your site determine which content (including HTML text and other data stored in the database, uploaded images, and any other asset files) is displayed on the pages of your site. The theme determines the HTML markup and CSS styling that wraps the content. Several basic themes are supplied with the core software; additional <em>contributed themes</em> can be downloaded separately from the <a href="https://www.drupal.org/project/project_theme">Download &amp; Extend page on drupal.org</a>, or you can create your own theme.{% endtrans %}</p>
+<h2>{% trans %}What is a base theme?{% endtrans %}</h2>
+<p>{% trans %}A base theme is a theme that is not meant to be used directly on a site, but instead acts as a scaffolding for building other themes. The core Classy theme is one example; other base themes can be downloaded from the <a href="https://www.drupal.org/project/project_theme">Download &amp; Extend page on drupal.org</a>.{% endtrans %}</p>
 <h2>{% trans %}What is a layout?{% endtrans %}</h2>
 <p>{% trans %}A <em>layout</em> is a template that defines where blocks and other pieces of content should be displayed. The core Layout Discovery module allows modules and themes to register layouts, and the core Layout Builder module provides a visual interface for placing fields and blocks in layouts for entity sub-types and individual entity items (see <a href="{{ entities }}">Managing content structure</a> for more on entities and fields).{% endtrans %}</p>
 <h2>{% trans %}Changing site appearance overview{% endtrans %}</h2>
diff --git a/core/modules/help_topics/help_topics/system.theme_uninstall.html.twig b/core/modules/help_topics/help_topics/system.theme_uninstall.html.twig
index d93cf79ad1..72bb7b8818 100644
--- a/core/modules/help_topics/help_topics/system.theme_uninstall.html.twig
+++ b/core/modules/help_topics/help_topics/system.theme_uninstall.html.twig
@@ -11,5 +11,5 @@ related:
 <ol>
   <li>{% trans %}In the <em>Manage</em> administrative menu, navigate to <a href="{{ themes_url }}"><em>Appearance</em></a>.{% endtrans %}</li>
   <li>{% trans %}Locate the theme that you want to uninstall, in the <em>Installed themes</em> section.{% endtrans %}</li>
-  <li>{% trans %}Click the <em>Uninstall</em> link to install the theme. If there is not an <em>Uninstall</em> link, the theme cannot be uninstalled because it is either being used as the site default theme or as the <em>Administration theme</em>.{% endtrans %}</li>
+  <li>{% trans %}Click the <em>Uninstall</em> link to install the theme. If there is not an <em>Uninstall</em> link, the theme cannot be uninstalled because it is either being used as the site default theme, being used as the <em>Administration theme</em>, or is the base theme for another installed theme.{% endtrans %}</li>
 </ol>