diff --git a/core/modules/system/templates/status-messages.html.twig b/core/modules/system/templates/status-messages.html.twig
index a2f3631..65206ed 100644
--- a/core/modules/system/templates/status-messages.html.twig
+++ b/core/modules/system/templates/status-messages.html.twig
@@ -40,9 +40,9 @@
         <h2 class="visually-hidden">{{ status_headings[type] }}</h2>
       {% endif %}
       {% if messages|length > 1 %}
-        <ul class="messages__list">
+        <ul>
           {% for message in messages %}
-            <li class="messages__item">{{ message }}</li>
+            <li>{{ message }}</li>
           {% endfor %}
         </ul>
       {% else %}
diff --git a/core/modules/system/templates/status-report.html.twig b/core/modules/system/templates/status-report.html.twig
index 89f12d4..4f6d841 100644
--- a/core/modules/system/templates/status-report.html.twig
+++ b/core/modules/system/templates/status-report.html.twig
@@ -17,7 +17,7 @@
  * @ingroup themeable
  */
 #}
-<table class="system-status-report">
+<table>
   <thead>
     <tr class="visually-hidden">
       <th>{{ 'Status'|t }}</th><th>{{ 'Component'|t }}</th><th>{{ 'Details'|t }}</th>
@@ -26,16 +26,16 @@
   <tbody>
   {% for requirement in requirements %}
     <tr class="{{ requirement.severity_status }}">
-      <td class="status-icon">
+      <td>
         <div title="{{ requirement.severity_title }}">
           <span class="visually-hidden">{{ requirement.severity_title }}</span>
         </div>
       </td>
-      <td class="status-title">{{ requirement.title }}</td>
-      <td class="status-value">
+      <td>{{ requirement.title }}</td>
+      <td>
         {{ requirement.value }}
         {% if requirement.description %}
-          <div class="description">{{ requirement.description }}</div>
+          <div>{{ requirement.description }}</div>
         {% endif %}
       </td>
     </tr>
diff --git a/core/modules/system/templates/system-admin-index.html.twig b/core/modules/system/templates/system-admin-index.html.twig
index 17f8150..881700f 100644
--- a/core/modules/system/templates/system-admin-index.html.twig
+++ b/core/modules/system/templates/system-admin-index.html.twig
@@ -15,10 +15,10 @@
  * @ingroup themeable
  */
 #}
-<div class="admin clearfix">
+<div>
   {{ system_compact_link }}
   {% for position, blocks in containers %}
-    <div class="{{ position }} clearfix">
+    <div>
       {% for block in blocks %}
         {{ block }}
       {% endfor %}
diff --git a/core/modules/system/templates/system-themes-page.html.twig b/core/modules/system/templates/system-themes-page.html.twig
index 6e65d76..a042051 100644
--- a/core/modules/system/templates/system-themes-page.html.twig
+++ b/core/modules/system/templates/system-themes-page.html.twig
@@ -40,7 +40,7 @@
       ]
     %}
     <div{{ theme_group.attributes.addClass(theme_group_classes) }}>
-      <h2 class="system-themes-list__header">{{ theme_group.title }}</h2>
+      <h2>{{ theme_group.title }}</h2>
       {% for theme in theme_group.themes %}
         {%
           set theme_classes = [
@@ -54,17 +54,17 @@
           {% if theme.screenshot %}
             {{ theme.screenshot }}
           {% endif %}
-          <div class="theme-info">
-            <h3 class="theme-info__header">
+          <div>
+            <h3>
               {{- theme.name }} {{ theme.version -}}
               {% if theme.notes %}
                 ({{ theme.notes|safe_join(', ') }})
               {%- endif -%}
             </h3>
-            <div class="theme-info__description">{{ theme.description }}</div>
+            <div>{{ theme.description }}</div>
             {# Display operation links if the theme is compatible. #}
             {% if theme.incompatible %}
-              <div class="incompatible">{{ theme.incompatible }}</div>
+              <div>{{ theme.incompatible }}</div>
             {% else %}
               {{ theme.operations }}
             {% endif %}
diff --git a/core/themes/classy/templates/select.html.twig b/core/themes/classy/templates/select.html.twig
new file mode 100644
index 0000000..21f32ac
--- /dev/null
+++ b/core/themes/classy/templates/select.html.twig
@@ -0,0 +1,15 @@
+{#
+/**
+ * @file
+ * Default theme implementation for a select element.
+ *
+ * Available variables:
+ * - attributes: HTML attributes for the select tag.
+ * - options: The option element children.
+ *
+ * @see template_preprocess_select()
+ *
+ * @ingroup themeable
+ */
+#}
+<select{{ attributes }}>{{ options }}</select>
diff --git a/core/themes/classy/templates/status-messages.html.twig b/core/themes/classy/templates/status-messages.html.twig
new file mode 100644
index 0000000..a2f3631
--- /dev/null
+++ b/core/themes/classy/templates/status-messages.html.twig
@@ -0,0 +1,57 @@
+{#
+/**
+ * @file
+ * Default theme implementation for status messages.
+ *
+ * Displays status, error, and warning messages, grouped by type.
+ *
+ * An invisible heading identifies the messages for assistive technology.
+ * Sighted users see a colored box. See http://www.w3.org/TR/WCAG-TECHS/H69.html
+ * for info.
+ *
+ * Add an ARIA label to the contentinfo area so that assistive technology
+ * user agents will better describe this landmark.
+ *
+ * Available variables:
+ * - message_list: List of messages to be displayed, grouped by type.
+ * - status_headings: List of all status types.
+ * - display: (optional) May have a value of 'status' or 'error' when only
+ *   displaying messages of that specific type.
+ * - attributes: HTML attributes for the element, including:
+ *   - class: HTML classes.
+ *
+ * @see template_preprocess_status_messages()
+ *
+ * @ingroup themeable
+ */
+#}
+{% for type, messages in message_list %}
+  {%
+    set classes = [
+      'messages',
+      'messages--' ~ type,
+    ]
+  %}
+  <div class="{{ attributes.addClass(classes).class }}" role="contentinfo" aria-label="{{ status_headings[type] }}">
+    {% if type == 'error' %}
+      <div role="alert">
+    {% endif %}
+      {% if status_headings[type] %}
+        <h2 class="visually-hidden">{{ status_headings[type] }}</h2>
+      {% endif %}
+      {% if messages|length > 1 %}
+        <ul class="messages__list">
+          {% for message in messages %}
+            <li class="messages__item">{{ message }}</li>
+          {% endfor %}
+        </ul>
+      {% else %}
+        {{ messages|first }}
+      {% endif %}
+    {% if type == 'error' %}
+      </div>
+    {% endif %}
+  </div>
+  {# Remove type specific classes. #}
+  {{ attributes.removeClass(classes) }}
+{% endfor %}
diff --git a/core/themes/classy/templates/status-report.html.twig b/core/themes/classy/templates/status-report.html.twig
new file mode 100644
index 0000000..89f12d4
--- /dev/null
+++ b/core/themes/classy/templates/status-report.html.twig
@@ -0,0 +1,44 @@
+{#
+/**
+ * @file
+ * Default theme implementation for the status report.
+ *
+ * Available variables:
+ * - requirements: Contains multiple requirement instances.
+ *   Each requirement contains:
+ *   - title: The title of the requirement.
+ *   - value: (optional) The requirement's status.
+ *   - description: (optional) The requirement's description.
+ *   - severity_title: The title of the severity.
+ *   - severity_status: Indicates the severity status.
+ *
+ * @see template_preprocess_status_report()
+ *
+ * @ingroup themeable
+ */
+#}
+<table class="system-status-report">
+  <thead>
+    <tr class="visually-hidden">
+      <th>{{ 'Status'|t }}</th><th>{{ 'Component'|t }}</th><th>{{ 'Details'|t }}</th>
+    </tr>
+  </thead>
+  <tbody>
+  {% for requirement in requirements %}
+    <tr class="{{ requirement.severity_status }}">
+      <td class="status-icon">
+        <div title="{{ requirement.severity_title }}">
+          <span class="visually-hidden">{{ requirement.severity_title }}</span>
+        </div>
+      </td>
+      <td class="status-title">{{ requirement.title }}</td>
+      <td class="status-value">
+        {{ requirement.value }}
+        {% if requirement.description %}
+          <div class="description">{{ requirement.description }}</div>
+        {% endif %}
+      </td>
+    </tr>
+  {% endfor %}
+  </tbody>
+</table>
diff --git a/core/themes/classy/templates/system-admin-index.html.twig b/core/themes/classy/templates/system-admin-index.html.twig
new file mode 100644
index 0000000..17f8150
--- /dev/null
+++ b/core/themes/classy/templates/system-admin-index.html.twig
@@ -0,0 +1,27 @@
+{#
+/**
+ * @file
+ * Default theme implementation for the admin index page.
+ *
+ * Available variables:
+ * - system_compact_link: Themed link to toggle compact view.
+ * - containers: A list of administrative containers keyed by position: left or
+ *   right. Each container in the list contains:
+ *   - blocks: A list of administrative blocks, rendered
+ *     through admin-block.html.twig.
+ *
+ * @see template_preprocess_system_admin_index()
+ *
+ * @ingroup themeable
+ */
+#}
+<div class="admin clearfix">
+  {{ system_compact_link }}
+  {% for position, blocks in containers %}
+    <div class="{{ position }} clearfix">
+      {% for block in blocks %}
+        {{ block }}
+      {% endfor %}
+    </div>
+  {% endfor %}
+</div>
diff --git a/core/themes/classy/templates/system-config-form.html.twig b/core/themes/classy/templates/system-config-form.html.twig
new file mode 100644
index 0000000..51e6fc7
--- /dev/null
+++ b/core/themes/classy/templates/system-config-form.html.twig
@@ -0,0 +1,17 @@
+{#
+/**
+ * @file
+ * Default theme implementation for a system settings form.
+ *
+ * This template will be used when a system config form specifies 'config_form'
+ * as its #theme callback.  Otherwise, by default, system config forms will be
+ * themed by theme_form(). This does not alter the appearance of a form at all,
+ * but is provided as a convenience for themers.
+ *
+ * Available variables:
+ * - form: The confirm form.
+ *
+ * @ingroup themeable
+ */
+#}
+{{ form }}
diff --git a/core/themes/classy/templates/system-themes-page.html.twig b/core/themes/classy/templates/system-themes-page.html.twig
new file mode 100644
index 0000000..6e65d76
--- /dev/null
+++ b/core/themes/classy/templates/system-themes-page.html.twig
@@ -0,0 +1,76 @@
+{#
+/**
+ * @file
+ * Default theme implementation for the Appearance page.
+ *
+ * Available variables:
+ * - attributes: HTML attributes for the main container.
+ * - theme_groups: A list of theme groups. Each theme group contains:
+ *   - attributes: HTML attributes specific to this theme group.
+ *   - title: Title for the theme group.
+ *   - state: State of the theme group, e.g. installed or uninstalled.
+ *   - themes: A list of themes within the theme group. Each theme contains:
+ *     - attributes: HTML attributes specific to this theme.
+ *     - screenshot: A screenshot representing the theme.
+ *     - description: Description of the theme.
+ *     - name: Theme name.
+ *     - version: The theme's version number.
+ *     - is_default: Boolean indicating whether the theme is the default theme
+ *       or not.
+ *     - is_admin: Boolean indicating whether the theme is the admin theme or
+ *       not.
+ *     - notes: Identifies what context this theme is being used in, e.g.,
+ *       default theme, admin theme.
+ *     - incompatible: Text describing any compatibility issues.
+ *     - operations: A list of operation links, e.g., Settings, Enable, Disable,
+ *       etc. these links should only be displayed if the theme is compatible.
+ *
+ * @see template_preprocess_system_themes_page()
+ *
+ * @ingroup themeable
+ */
+#}
+<div{{ attributes }}>
+  {% for theme_group in theme_groups %}
+    {%
+      set theme_group_classes = [
+        'system-themes-list',
+        'system-themes-list-' ~ theme_group.state,
+        'clearfix',
+      ]
+    %}
+    <div{{ theme_group.attributes.addClass(theme_group_classes) }}>
+      <h2 class="system-themes-list__header">{{ theme_group.title }}</h2>
+      {% for theme in theme_group.themes %}
+        {%
+          set theme_classes = [
+            theme.is_default ? 'theme-default',
+            theme.is_admin ? 'theme-admin',
+            'theme-selector',
+            'clearfix',
+          ]
+        %}
+        <div{{ theme.attributes.addClass(theme_classes) }}>
+          {% if theme.screenshot %}
+            {{ theme.screenshot }}
+          {% endif %}
+          <div class="theme-info">
+            <h3 class="theme-info__header">
+              {{- theme.name }} {{ theme.version -}}
+              {% if theme.notes %}
+                ({{ theme.notes|safe_join(', ') }})
+              {%- endif -%}
+            </h3>
+            <div class="theme-info__description">{{ theme.description }}</div>
+            {# Display operation links if the theme is compatible. #}
+            {% if theme.incompatible %}
+              <div class="incompatible">{{ theme.incompatible }}</div>
+            {% else %}
+              {{ theme.operations }}
+            {% endif %}
+          </div>
+        </div>
+      {% endfor %}
+    </div>
+  {% endfor %}
+</div>