nicoloye
--- a/core/modules/contextual/js/contextual.js
+++ b/core/modules/contextual/js/contextual.js
      // Perform an AJAX request to let the server render the contextual links
      // for each of the placeholders.
      if (uncachedIDs.length > 0) {
+        const controllerUrl = `contextual/render?theme=${drupalSettings.contextual.theme}`;
        $.ajax({
-          url: Drupal.url('contextual/render'),
+          url: Drupal.url(controllerUrl),
          type: 'POST',
          data: { 'ids[]': uncachedIDs, 'tokens[]': uncachedTokens },
          dataType: 'json',
          success(results) {
            Object.entries(results).forEach(([contextualID, html]) => {
              // Store the metadata.
              storage.setItem(`Drupal.contextual.${contextualID}`, html);
--- a/core/modules/contextual/js/contextual.js
+++ b/core/modules/contextual/js/contextual.js
      // Perform an AJAX request to let the server render the contextual links
      // for each of the placeholders.
      if (uncachedIDs.length > 0) {
+        const controllerUrl = `contextual/render?theme=${drupalSettings.contextual.theme}`;
        $.ajax({
-          url: Drupal.url('contextual/render'),
+          url: Drupal.url(controllerUrl),
          type: 'POST',
          data: { 'ids[]': uncachedIDs, 'tokens[]': uncachedTokens },
          dataType: 'json',
          success(results) {
            Object.entries(results).forEach(([contextualID, html]) => {
              // Store the metadata.
              storage.setItem(`Drupal.contextual.${contextualID}`, html);
--- a/core/modules/contextual/tests/src/Kernel/ContextualUnitTest.php
+++ b/core/modules/contextual/tests/src/Kernel/ContextualUnitTest.php
    $this->assertSame($links, _contextual_id_to_links($id));
  }

+  /**
+   * Tests the placeholder of contextual links in a specific theme.
+   *
+   * @param array $links
+   *   The #contextual_links property value array.
+   * @param string $id
+   *   The serialized representation of the passed links.
+   * @param string $theme
+   *   The name of the theme the placeholder should pass to the controller.
+   *
+   * @covers \Drupal\contextual\Element\ContextualLinksPlaceholder::preRenderPlaceholder
--- a/core/modules/contextual/tests/src/Kernel/ContextualUnitTest.php
+++ b/core/modules/contextual/tests/src/Kernel/ContextualUnitTest.php
    $this->assertSame($links, _contextual_id_to_links($id));
  }

+  /**
+   * Tests the placeholder of contextual links in a specific theme.
+   *
+   * @param array $links
+   *   The #contextual_links property value array.
+   * @param string $id
+   *   The serialized representation of the passed links.
+   * @param string $theme
+   *   The name of the theme the placeholder should pass to the controller.
+   *
+   * @covers \Drupal\contextual\Element\ContextualLinksPlaceholder::preRenderPlaceholder
+   *
+   * @dataProvider contextualLinksDataProvider