diff -u b/core/lib/Drupal/Core/Theme/ThemeNegotiatorInterface.php b/core/lib/Drupal/Core/Theme/ThemeNegotiatorInterface.php
--- b/core/lib/Drupal/Core/Theme/ThemeNegotiatorInterface.php
+++ b/core/lib/Drupal/Core/Theme/ThemeNegotiatorInterface.php
@@ -12,11 +12,18 @@
 /**
  * Defines an interface for classes which determine the active theme.
  *
- * In order to set the active theme you have to create a new service tagged
- * with 'theme_negotiator', see user.services.yml as example. The only method
- * this service has to implement is determineActiveTheme. Return the name of the
- * theme or NULL, if other negotiators like the configured default one should
- * kick in instead.
+ * To set the active theme, create a new service tagged with 'theme_negotiator'
+ * (see user.services.yml for an example). The only method this service needs
+ * to implement is determineActiveTheme. Return the name of the theme, or NULL
+ * if other negotiators like the configured default one should kick in instead.
+ *
+ * If you are setting a theme which is closely tied to the functionality of a
+ * particular page or set of pages (such that the page might not function
+ * correctly if a different theme is used), make sure to set the priority on
+ * the service to a high number so that it is not accidentally overridden by
+ * other theme negotiators. By convention, a priority of "1000" is used in
+ * these cases; see \Drupal\Core\Theme\AjaxBasePageNegotiator and
+ * core.services.yml for an example.
  */
 interface ThemeNegotiatorInterface {