diff --git a/core/modules/aggregator/aggregator.pages.inc b/core/modules/aggregator/aggregator.pages.inc
index 8938e95..b6d4062 100644
--- a/core/modules/aggregator/aggregator.pages.inc
+++ b/core/modules/aggregator/aggregator.pages.inc
@@ -376,7 +376,12 @@ function aggregator_page_rss() {
   }
 
   $feeds = $result->fetchAll();
-  return theme('aggregator_page_rss', array('feeds' => $feeds, 'category' => $category));
+  $aggregator_page_rss = array(
+    '#theme' => 'aggregator_page_rss',
+    '#feeds' => $feeds,
+    '#category' => $category,
+  );
+  return drupal_render($aggregator_page_rss);
 }
 
 /**
@@ -448,7 +453,11 @@ function aggregator_page_opml($cid = NULL) {
   }
 
   $feeds = $result->fetchAll();
-  return theme('aggregator_page_opml', array('feeds' => $feeds));
+  $aggregator_page_opml = array(
+    '#theme' => 'aggregator_page_opml',
+    '#feeds' => $feeds,
+  );
+  return drupal_render($aggregator_page_opml);
 }
 
 /**
@@ -523,14 +532,16 @@ function template_preprocess_aggregator_summary_item(&$variables) {
       'class' => array('feed-item-url',),
     ),
   ));
-  $variables['item_age'] = theme('datetime', array(
-    'attributes' => array(
+  $datetime = array(
+    '#theme' => 'datetime',
+    '#attributes' => array(
       'datetime' => format_date($item->timestamp->value, 'html_datetime', '', 'UTC'),
       'class' => array('feed-item-age',),
     ),
-    'text' => t('%age old', array('%age' => format_interval(REQUEST_TIME - $item->timestamp->value))),
-    'html' => TRUE,
-  ));
+    '#text' => t('%age old', array('%age' => format_interval(REQUEST_TIME - $item->timestamp->value))),
+    '#html' => TRUE,
+  );
+  $variables['item_age'] = drupal_render($datetime);
 }
 
 /**
@@ -546,10 +557,20 @@ function template_preprocess_aggregator_summary_item(&$variables) {
 function template_preprocess_aggregator_feed_source(&$variables) {
   $feed = $variables['aggregator_feed'];
 
-  $variables['source_icon'] = theme('feed_icon', array('url' => $feed->url->value, 'title' => t('!title feed', array('!title' => $feed->label()))));
+  $feed_icon = array(
+    '#theme' => 'feed_icon',
+    '#url' => $feed->url->value,
+    '#title' => t('!title feed', array('!title' => $feed->label())),
+  );
+  $variables['source_icon'] = drupal_render($feed_icon);
 
   if (!empty($feed->image->value) && $feed->label() && !empty($feed->link->value)) {
-    $variables['source_image'] = l(theme('image', array('path' => $feed->image->value, 'alt' => $feed->title->value)), $feed->link->value, array('html' => TRUE, 'attributes' => array('class' => 'feed-image')));
+    $image = array(
+      '#theme' => 'image',
+      '#path' => $feed->image->value,
+      '#alt' => $feed->title->value,
+    );
+    $variables['source_image'] = l($image, $feed->link->value, array('html' => TRUE, 'attributes' => array('class' => 'feed-image')));
   }
   else {
     $variables['source_image'] = '';
diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Plugin/Block/AggregatorCategoryBlock.php b/core/modules/aggregator/lib/Drupal/aggregator/Plugin/Block/AggregatorCategoryBlock.php
index 423c903..c9fe040 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Plugin/Block/AggregatorCategoryBlock.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Plugin/Block/AggregatorCategoryBlock.php
@@ -68,17 +68,30 @@ public function build() {
     $id = $this->getPluginId();
     if ($category = db_query('SELECT cid, title, block FROM {aggregator_category} WHERE cid = :cid', array(':cid' => $id))->fetchObject()) {
       $result = db_query_range('SELECT i.* FROM {aggregator_category_item} ci LEFT JOIN {aggregator_item} i ON ci.iid = i.iid WHERE ci.cid = :cid ORDER BY i.timestamp DESC, i.iid DESC', 0, $this->configuration['block_count'], array(':cid' => $category->cid));
-      $read_more = theme('more_link', array('url' => 'aggregator/categories/' . $category->cid, 'title' => t("View this category's recent news.")));
+      $more_link = array(
+        '#theme' => 'more_link',
+        '#url' => 'aggregator/categories/' . $category->cid,
+        '#title' => t("View this category's recent news."),
+      );
+      $read_more = drupal_render($more_link);
 
       $items = array();
       foreach ($result as $item) {
-        $items[] = theme('aggregator_block_item', array('item' => $item));
+        $aggregator_block_item = array(
+          '#theme' => 'aggregator_block_item',
+          '#item' => $item,
+        );
+        $items[] = drupal_render($aggregator_block_item);
       }
 
       // Only display the block if there are items to show.
       if (count($items) > 0) {
+        $item_list = array(
+          '#theme' => 'item_list',
+          '#items' => $items,
+        );
         return array(
-          '#children' => theme('item_list', array('items' => $items)) . $read_more,
+          '#children' => drupal_render($item_list) . $read_more,
         );
       }
       return array();
diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Plugin/Block/AggregatorFeedBlock.php b/core/modules/aggregator/lib/Drupal/aggregator/Plugin/Block/AggregatorFeedBlock.php
index 3cd519f..4b002a2 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Plugin/Block/AggregatorFeedBlock.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Plugin/Block/AggregatorFeedBlock.php
@@ -69,16 +69,28 @@ public function build() {
     list(, $id) = explode(':', $this->getPluginId());
     if ($feed = db_query('SELECT fid, title, block FROM {aggregator_feed} WHERE block <> 0 AND fid = :fid', array(':fid' => $id))->fetchObject()) {
       $result = db_query_range("SELECT * FROM {aggregator_item} WHERE fid = :fid ORDER BY timestamp DESC, iid DESC", 0, $this->configuration['block_count'], array(':fid' => $id));
-      $read_more = theme('more_link', array('url' => 'aggregator/sources/' . $feed->fid, 'title' => t("View this feed's recent news.")));
-
+      $more_link = array(
+        '#theme' => 'more_link',
+        '#url' => 'aggregator/sources/' . $feed->fid,
+        '#title' => t("View this feed's recent news."),
+      );
+      $read_more = drupal_render($more_link);
       $items = array();
       foreach ($result as $item) {
-        $items[] = theme('aggregator_block_item', array('item' => $item));
+        $aggregator_block_item = array(
+          '#theme' => 'aggregator_block_item',
+          '#item' => $item,
+        );
+        $items[] = drupal_render($aggregator_block_item);
       }
       // Only display the block if there are items to show.
       if (count($items) > 0) {
+        $item_list = array(
+          '#theme' => 'item_list',
+          '#items' => $items,
+        );
         return array(
-          '#children' => theme('item_list', array('items' => $items)) . $read_more,
+          '#children' => drupal_render($item_list) . $read_more,
         );
       }
     }