diff --git a/core/modules/system/templates/admin-block-content.html.twig b/core/modules/system/templates/admin-block-content.html.twig
index e70e565..025ba9c 100644
--- a/core/modules/system/templates/admin-block-content.html.twig
+++ b/core/modules/system/templates/admin-block-content.html.twig
@@ -16,14 +16,9 @@
  * @ingroup themeable
  */
 #}
-{%
-  set classes = [
-    'admin-list',
-    compact ? 'compact',
-  ]
-%}
+
 {% if content %}
-  <dl{{ attributes.addClass(classes) }}>
+  <dl{{ attributes }}>
     {% for item in content %}
       <dt>{{ item.link }}</dt>
       {% if item.description %}
diff --git a/core/modules/system/templates/admin-block.html.twig b/core/modules/system/templates/admin-block.html.twig
index 20aa000..542fe04 100644
--- a/core/modules/system/templates/admin-block.html.twig
+++ b/core/modules/system/templates/admin-block.html.twig
@@ -14,13 +14,13 @@
  * @ingroup themeable
  */
 #}
-<div class="admin-panel">
+<div>
   {% if block.title %}
     <h3>{{ block.title }}</h3>
   {% endif %}
   {% if block.content %}
-    <div class="body">{{ block.content }}</div>
+    <div>{{ block.content }}</div>
   {% elseif block.description %}
-    <div class="description">{{ block.description }}</div>
+    <div>{{ block.description }}</div>
   {% endif %}
 </div>
diff --git a/core/modules/system/templates/admin-page.html.twig b/core/modules/system/templates/admin-page.html.twig
index 038b1af..8ed590a 100644
--- a/core/modules/system/templates/admin-page.html.twig
+++ b/core/modules/system/templates/admin-page.html.twig
@@ -14,10 +14,10 @@
  * @ingroup themeable
  */
 #}
-<div class="clearfix">
+<div>
   {{ system_compact_link }}
   {% for container in containers %}
-    <div class="layout-column half">
+    <div>
       {% for block in container.blocks %}
         {{ block }}
       {% endfor %}
diff --git a/core/modules/system/templates/block--system-branding-block.html.twig b/core/modules/system/templates/block--system-branding-block.html.twig
index 4cf0f1a..f045f5d 100644
--- a/core/modules/system/templates/block--system-branding-block.html.twig
+++ b/core/modules/system/templates/block--system-branding-block.html.twig
@@ -22,11 +22,11 @@
     </a>
   {% endif %}
   {% if site_name %}
-    <div class="site-name">
+    <div>
       <a href="{{ url('<front>') }}" title="{{ 'Home'|t }}" rel="home">{{ site_name }}</a>
     </div>
   {% endif %}
   {% if site_slogan %}
-    <div class="site-slogan">{{ site_slogan }}</div>
+    <div>{{ site_slogan }}</div>
   {% endif %}
 {% endblock %}
diff --git a/core/modules/system/templates/block--system-menu-block.html.twig b/core/modules/system/templates/block--system-menu-block.html.twig
index dff7e39..72cc966 100644
--- a/core/modules/system/templates/block--system-menu-block.html.twig
+++ b/core/modules/system/templates/block--system-menu-block.html.twig
@@ -40,19 +40,12 @@
  * @ingroup themeable
  */
 #}
-{%
-  set classes = [
-    'block',
-    'block-menu',
-    'navigation',
-    'menu--' ~ derivative_plugin_id|clean_class,
-  ]
-%}
+
 {% set heading_id = attributes.id ~ '-menu'|clean_id %}
-<nav{{ attributes.addClass(classes) }} role="navigation" aria-labelledby="{{ heading_id }}">
+<nav{{ attributes }} role="navigation" aria-labelledby="{{ heading_id }}">
   {# Label. If not displayed, we still provide it for screen readers. #}
   {% if not configuration.label_display %}
-    {% set title_attributes = title_attributes.addClass('visually-hidden') %}
+    {% set title_attributes = title_attributes %}
   {% endif %}
   {{ title_prefix }}
   <h2 id="{{ heading_id }}"{{ title_attributes }}>{{ configuration.label }}</h2>
diff --git a/core/themes/classy/templates/admin-block-content.html.twig b/core/themes/classy/templates/admin-block-content.html.twig
new file mode 100644
index 0000000..e70e565
--- /dev/null
+++ b/core/themes/classy/templates/admin-block-content.html.twig
@@ -0,0 +1,34 @@
+{#
+/**
+ * @file
+ * Default theme implementation for the content of an administrative block.
+ *
+ * Available variables:
+ * - content: A list containing information about the block. Each element
+ *   of the array represents an administrative menu item, and must at least
+ *   contain the keys 'title', 'link_path', and 'localized_options', which are
+ *   passed to l(). A 'description' key may also be provided.
+ * - attributes: HTML attributes to be added to the element.
+ * - compact: Boolean indicating whether compact mode is turned on or not.
+ *
+ * @see template_preprocess_admin_block_content()
+ *
+ * @ingroup themeable
+ */
+#}
+{%
+  set classes = [
+    'admin-list',
+    compact ? 'compact',
+  ]
+%}
+{% if content %}
+  <dl{{ attributes.addClass(classes) }}>
+    {% for item in content %}
+      <dt>{{ item.link }}</dt>
+      {% if item.description %}
+        <dd>{{ item.description }}</dd>
+      {% endif %}
+    {% endfor %}
+  </dl>
+{% endif %}
diff --git a/core/themes/classy/templates/admin-block.html.twig b/core/themes/classy/templates/admin-block.html.twig
new file mode 100644
index 0000000..20aa000
--- /dev/null
+++ b/core/themes/classy/templates/admin-block.html.twig
@@ -0,0 +1,26 @@
+{#
+/**
+ * @file
+ * Default theme implementation for an administrative block.
+ *
+ * Available variables:
+ * - block: An array of information about the block, including:
+ *   - show: A flag indicating if the block should be displayed.
+ *   - title: The block title.
+ *   - content: (optional) The content of the block.
+ *   - description: (optional) A description of the block.
+ *     (Description should only be output if content is not available).
+ *
+ * @ingroup themeable
+ */
+#}
+<div class="admin-panel">
+  {% if block.title %}
+    <h3>{{ block.title }}</h3>
+  {% endif %}
+  {% if block.content %}
+    <div class="body">{{ block.content }}</div>
+  {% elseif block.description %}
+    <div class="description">{{ block.description }}</div>
+  {% endif %}
+</div>
diff --git a/core/themes/classy/templates/admin-page.html.twig b/core/themes/classy/templates/admin-page.html.twig
new file mode 100644
index 0000000..038b1af
--- /dev/null
+++ b/core/themes/classy/templates/admin-page.html.twig
@@ -0,0 +1,26 @@
+{#
+/**
+ * @file
+ * Default theme implementation for an administrative page.
+ *
+ * Available variables:
+ * - system_compact_link: Themed link to toggle compact view.
+ * - containers: An list of administrative blocks keyed by position: left or
+ *   right. Contains:
+ *   - blocks: A list of blocks within a container.
+ *
+ * @see template_preprocess_admin_page()
+ *
+ * @ingroup themeable
+ */
+#}
+<div class="clearfix">
+  {{ system_compact_link }}
+  {% for container in containers %}
+    <div class="layout-column half">
+      {% for block in container.blocks %}
+        {{ block }}
+      {% endfor %}
+    </div>
+  {% endfor %}
+</div>
diff --git a/core/themes/classy/templates/block--system-branding-block.html.twig b/core/themes/classy/templates/block--system-branding-block.html.twig
new file mode 100644
index 0000000..4cf0f1a
--- /dev/null
+++ b/core/themes/classy/templates/block--system-branding-block.html.twig
@@ -0,0 +1,32 @@
+{% extends "@block/block.html.twig" %}
+{#
+/**
+ * @file
+ * Default theme implementation for a branding block.
+ *
+ * Each branding element variable (logo, name, slogan) is only available if
+ * enabled in the block configuration.
+ *
+ * Available variables:
+ * - site_logo: Logo for site as defined in Appearance or theme settings.
+ * - site_name: Name for site as defined in Site information settings.
+ * - site_slogan: Slogan for site as defined in Site information settings.
+ *
+ * @ingroup themeable
+ */
+#}
+{% block content %}
+  {% if site_logo %}
+    <a href="{{ url('<front>') }}" title="{{ 'Home'|t }}" rel="home" class="site-logo">
+      <img src="{{ site_logo }}" alt="{{ 'Home'|t }}" />
+    </a>
+  {% endif %}
+  {% if site_name %}
+    <div class="site-name">
+      <a href="{{ url('<front>') }}" title="{{ 'Home'|t }}" rel="home">{{ site_name }}</a>
+    </div>
+  {% endif %}
+  {% if site_slogan %}
+    <div class="site-slogan">{{ site_slogan }}</div>
+  {% endif %}
+{% endblock %}
diff --git a/core/themes/classy/templates/block--system-menu-block.html.twig b/core/themes/classy/templates/block--system-menu-block.html.twig
new file mode 100644
index 0000000..dff7e39
--- /dev/null
+++ b/core/themes/classy/templates/block--system-menu-block.html.twig
@@ -0,0 +1,65 @@
+{#
+/**
+ * @file
+ * Default theme implementation for a menu block.
+ *
+ * Available variables:
+ * - plugin_id: The ID of the block implementation.
+ * - label: The configured label of the block if visible.
+ * - configuration: A list of the block's configuration values.
+ *   - label: The configured label for the block.
+ *   - label_display: The display settings for the label.
+ *   - module: The module that provided this block plugin.
+ *   - cache: The cache settings.
+ *   - Block plugin specific settings will also be stored here.
+ * - block - The full block entity.
+ *   - label_hidden: The hidden block title value if the block was
+ *     configured to hide the title ('label' is empty in this case).
+ *   - module: The module that generated the block.
+ *   - delta: An ID for the block, unique within each module.
+ *   - region: The block region embedding the current block.
+ * - content: The content of this block.
+ * - attributes: HTML attributes for the containing element.
+ *   - id: A valid HTML ID and guaranteed unique.
+ * - title_attributes: HTML attributes for the title element.
+ * - content_attributes: HTML attributes for the content element.
+ * - title_prefix: Additional output populated by modules, intended to be
+ *   displayed in front of the main title tag that appears in the template.
+ * - title_suffix: Additional output populated by modules, intended to be
+ *   displayed after the main title tag that appears in the template.
+ *
+ * Headings should be used on navigation menus that consistently appear on
+ * multiple pages. When this menu block's label is configured to not be
+ * displayed, it is automatically made invisible using the 'visually-hidden' CSS
+ * class, which still keeps it visible for screen-readers and assistive
+ * technology. Headings allow screen-reader and keyboard only users to navigate
+ * to or skip the links.
+ * See http://juicystudio.com/article/screen-readers-display-none.php and
+ * http://www.w3.org/TR/WCAG-TECHS/H42.html for more information.
+ *
+ * @ingroup themeable
+ */
+#}
+{%
+  set classes = [
+    'block',
+    'block-menu',
+    'navigation',
+    'menu--' ~ derivative_plugin_id|clean_class,
+  ]
+%}
+{% set heading_id = attributes.id ~ '-menu'|clean_id %}
+<nav{{ attributes.addClass(classes) }} role="navigation" aria-labelledby="{{ heading_id }}">
+  {# Label. If not displayed, we still provide it for screen readers. #}
+  {% if not configuration.label_display %}
+    {% set title_attributes = title_attributes.addClass('visually-hidden') %}
+  {% endif %}
+  {{ title_prefix }}
+  <h2 id="{{ heading_id }}"{{ title_attributes }}>{{ configuration.label }}</h2>
+  {{ title_suffix }}
+
+  {# Menu. #}
+  {% block content %}
+    {{ content }}
+  {% endblock %}
+</nav>