diff --git a/core/modules/block/tests/themes/block_test_theme/block_test_theme.info b/core/modules/block/tests/themes/block_test_theme/block_test_theme.info
new file mode 100644
index 0000000000000000000000000000000000000000..6b15fe5ed388272e7f41051250495e16efbd9825
--- /dev/null
+++ b/core/modules/block/tests/themes/block_test_theme/block_test_theme.info
@@ -0,0 +1,14 @@
+name = Block test theme
+description = Theme for testing the block system
+core = 8.x
+hidden = TRUE
+
+regions[sidebar_first] = Left sidebar
+regions_hidden[]  = sidebar_first
+regions[sidebar_second] = Right sidebar
+regions_hidden[]  = sidebar_second
+regions[content] = Content
+regions[header] = Header
+regions[footer] = Footer
+regions[highlighted] = Highlighted
+regions[help] = Help
diff --git a/core/modules/block/tests/themes/block_test_theme/page.tpl.php b/core/modules/block/tests/themes/block_test_theme/page.tpl.php
new file mode 100644
index 0000000000000000000000000000000000000000..ba72882ef98b51a11b3e615ca1a585efc2f2654d
--- /dev/null
+++ b/core/modules/block/tests/themes/block_test_theme/page.tpl.php
@@ -0,0 +1,78 @@
+<?php
+
+/**
+ * @file
+ * Custom theme implementation to display a single Drupal page without
+ * sidebars. The sidebars are hidden by regions_hidden for this theme, so
+ * the default page.tpl.php will not work without throwing exceptions.
+ */
+?>
+
+  <div id="page-wrapper"><div id="page">
+
+    <div id="header"><div class="section clearfix">
+
+      <?php if ($logo): ?>
+        <a href="<?php print $front_page; ?>" title="<?php print t('Home'); ?>" rel="home" id="logo">
+          <img src="<?php print $logo; ?>" alt="<?php print t('Home'); ?>" />
+        </a>
+      <?php endif; ?>
+
+      <?php if ($site_name || $site_slogan): ?>
+        <div id="name-and-slogan">
+          <?php if ($site_name): ?>
+            <?php if ($title): ?>
+              <div id="site-name"><strong>
+                <a href="<?php print $front_page; ?>" title="<?php print t('Home'); ?>" rel="home"><span><?php print $site_name; ?></span></a>
+              </strong></div>
+            <?php else: /* Use h1 when the content title is empty */ ?>
+              <h1 id="site-name">
+                <a href="<?php print $front_page; ?>" title="<?php print t('Home'); ?>" rel="home"><span><?php print $site_name; ?></span></a>
+              </h1>
+            <?php endif; ?>
+          <?php endif; ?>
+
+          <?php if ($site_slogan): ?>
+            <div id="site-slogan"><?php print $site_slogan; ?></div>
+          <?php endif; ?>
+        </div> <!-- /#name-and-slogan -->
+      <?php endif; ?>
+
+      <?php print render($page['header']); ?>
+
+    </div></div> <!-- /.section, /#header -->
+
+    <?php if ($main_menu || $secondary_menu): ?>
+      <div id="navigation"><div class="section">
+        <?php print theme('links__system_main_menu', array('links' => $main_menu, 'attributes' => array('id' => 'main-menu', 'class' => array('links', 'inline', 'clearfix')), 'heading' => t('Main menu'))); ?>
+        <?php print theme('links__system_secondary_menu', array('links' => $secondary_menu, 'attributes' => array('id' => 'secondary-menu', 'class' => array('links', 'inline', 'clearfix')), 'heading' => t('Secondary menu'))); ?>
+      </div></div> <!-- /.section, /#navigation -->
+    <?php endif; ?>
+
+    <?php if ($breadcrumb): ?>
+      <div id="breadcrumb"><?php print $breadcrumb; ?></div>
+    <?php endif; ?>
+
+    <?php print $messages; ?>
+
+    <div id="main-wrapper"><div id="main" class="clearfix">
+
+      <div id="content" class="column"><div class="section">
+        <?php if ($page['highlighted']): ?><div id="highlighted"><?php print render($page['highlighted']); ?></div><?php endif; ?>
+        <a id="main-content"></a>
+        <?php print render($title_prefix); ?>
+        <?php if ($title): ?><h1 class="title" id="page-title"><?php print $title; ?></h1><?php endif; ?>
+        <?php print render($title_suffix); ?>
+        <?php if ($tabs = render($tabs)): ?><div class="tabs"><?php print $tabs; ?></div><?php endif; ?>
+        <?php print render($page['help']); ?>
+        <?php if ($action_links): ?><ul class="action-links"><?php print render($action_links); ?></ul><?php endif; ?>
+        <?php print render($page['content']); ?>
+        <?php print $feed_icons; ?>
+      </div></div> <!-- /.section, /#content -->
+    </div></div> <!-- /#main, /#main-wrapper -->
+
+    <div id="footer"><div class="section">
+      <?php print render($page['footer']); ?>
+    </div></div> <!-- /.section, /#footer -->
+
+  </div></div> <!-- /#page, /#page-wrapper -->
diff --git a/core/modules/simpletest/tests/themes/test_theme/template.php b/core/modules/simpletest/tests/themes/test_theme/template.php
new file mode 100644
index 0000000000000000000000000000000000000000..ef8118a6da947674484fef05351ce9dd3cf03769
--- /dev/null
+++ b/core/modules/simpletest/tests/themes/test_theme/template.php
@@ -0,0 +1,21 @@
+<?php
+
+/**
+ * Tests a theme overriding a suggestion of a base theme hook.
+ */
+function test_theme_breadcrumb__suggestion($variables) {
+  // Tests that preprocess functions for the base theme hook get called even
+  // when the suggestion has an implementation.
+  return 'test_theme_breadcrumb__suggestion: ' . $variables['theme_test_preprocess_breadcrumb'];
+}
+
+/**
+ * Tests a theme implementing an alter hook.
+ *
+ * The confusing function name here is due to this being an implementation of
+ * the alter hook invoked when the 'theme_test' module calls
+ * drupal_alter('theme_test_alter').
+ */
+function test_theme_theme_test_alter_alter(&$data) {
+  $data = 'test_theme_theme_test_alter_alter was invoked';
+}
diff --git a/core/modules/simpletest/tests/themes/test_theme/test_theme.info b/core/modules/simpletest/tests/themes/test_theme/test_theme.info
new file mode 100644
index 0000000000000000000000000000000000000000..c32fe57a629e86b1a85835e1e5c6f27d064d1d82
--- /dev/null
+++ b/core/modules/simpletest/tests/themes/test_theme/test_theme.info
@@ -0,0 +1,16 @@
+name = Test theme
+description = Theme for testing the theme system
+core = 8.x
+hidden = TRUE
+
+; Normally, themes may list CSS files like this, and if they exist in the theme
+; folder, then they get added to the page. If they have the same file name as a
+; module CSS file, then the theme's version overrides the module's version, so
+; that the module's version is not added to the page. Additionally, a theme may
+; have an entry like this one, without having the corresponding CSS file in the
+; theme's folder, and in this case, it just stops the module's version from
+; being loaded, and does not replace it with an alternate version. We have this
+; here in order for a test to ensure that this correctly prevents the module
+; version from being loaded, and that errors aren't caused by the lack of this
+; file within the theme folder.
+stylesheets[all][] = system.base.css
diff --git a/core/modules/simpletest/tests/themes/test_theme/theme_test.template_test.tpl.php b/core/modules/simpletest/tests/themes/test_theme/theme_test.template_test.tpl.php
new file mode 100644
index 0000000000000000000000000000000000000000..d4409bf16e0a3962ad22e5f1fda1d629194fb7bd
--- /dev/null
+++ b/core/modules/simpletest/tests/themes/test_theme/theme_test.template_test.tpl.php
@@ -0,0 +1,2 @@
+<!-- Output for Theme API test -->
+Success: Template overridden.
diff --git a/core/modules/update/tests/themes/update_test_basetheme/update_test_basetheme.info b/core/modules/update/tests/themes/update_test_basetheme/update_test_basetheme.info
new file mode 100644
index 0000000000000000000000000000000000000000..c8550b5bc8c868b5759b7f5d99e0d285fc94479c
--- /dev/null
+++ b/core/modules/update/tests/themes/update_test_basetheme/update_test_basetheme.info
@@ -0,0 +1,4 @@
+name = Update test base theme
+description = Test theme which acts as a base theme for other test subthemes.
+core = 8.x
+hidden = TRUE
diff --git a/core/modules/update/tests/themes/update_test_subtheme/update_test_subtheme.info b/core/modules/update/tests/themes/update_test_subtheme/update_test_subtheme.info
new file mode 100644
index 0000000000000000000000000000000000000000..c783dc28b1e8e4a4fe706111e47f5d647b204cec
--- /dev/null
+++ b/core/modules/update/tests/themes/update_test_subtheme/update_test_subtheme.info
@@ -0,0 +1,5 @@
+name = Update test subtheme
+description = Test theme which uses update_test_basetheme as the base theme.
+core = 8.x
+base theme = update_test_basetheme
+hidden = TRUE