diff --git a/gtk/gtkcsscustomproperty.c b/gtk/gtkcsscustomproperty.c
index 017a8a22e1..4c0437983f 100644
--- a/gtk/gtkcsscustomproperty.c
+++ b/gtk/gtkcsscustomproperty.c
@@ -211,6 +211,8 @@ gtk_css_custom_property_create_initial_value (GParamSpec *pspec)
  * </note>
  *
  * Since: 3.0
+ *
+ * Deprecated: 3.8: Code should use the default properties provided by CSS.
  **/
 void
 gtk_theming_engine_register_property (const gchar            *name_space,
@@ -260,6 +262,8 @@ gtk_theming_engine_register_property (const gchar            *name_space,
  * a theming engine, you want to use that function instead.
  *
  * Since: 3.0
+ *
+ * Deprecated: 3.8: Code should use the default properties provided by CSS.
  **/
 void
 gtk_style_properties_register_property (GtkStylePropertyParser  parse_func,
@@ -303,6 +307,9 @@ gtk_style_properties_register_property (GtkStylePropertyParser  parse_func,
  * Returns: %TRUE if the property is registered, %FALSE otherwise
  *
  * Since: 3.0
+ *
+ * Deprecated: 3.8: This code could only look up custom properties and
+ *     those are deprecated.
  **/
 gboolean
 gtk_style_properties_lookup_property (const gchar             *property_name,
diff --git a/gtk/gtkstyleproperties.h b/gtk/gtkstyleproperties.h
index eb8ff708f2..7237361e2d 100644
--- a/gtk/gtkstyleproperties.h
+++ b/gtk/gtkstyleproperties.h
@@ -66,8 +66,10 @@ typedef gboolean (* GtkStylePropertyParser) (const gchar  *string,
 GType gtk_style_properties_get_type (void) G_GNUC_CONST;
 
 /* Next 2 are implemented in gtkcsscustomproperty.c */
+GDK_DEPRECATED_IN_3_8
 void     gtk_style_properties_register_property (GtkStylePropertyParser  parse_func,
                                                  GParamSpec             *pspec);
+GDK_DEPRECATED_IN_3_8
 gboolean gtk_style_properties_lookup_property   (const gchar             *property_name,
                                                  GtkStylePropertyParser  *parse_func,
                                                  GParamSpec             **pspec);
diff --git a/gtk/gtkthemingengine.h b/gtk/gtkthemingengine.h
index 8ea017d5a3..0bdc3a13cf 100644
--- a/gtk/gtkthemingengine.h
+++ b/gtk/gtkthemingengine.h
@@ -182,6 +182,7 @@ struct _GtkThemingEngineClass
 GType gtk_theming_engine_get_type (void) G_GNUC_CONST;
 
 /* function implemented in gtkcsscustomproperty.c */
+GDK_DEPRECATED_IN_3_8
 void gtk_theming_engine_register_property (const gchar            *name_space,
                                            GtkStylePropertyParser  parse_func,
                                            GParamSpec             *pspec);