diff --git a/gfx/public/nsThemeConstants.h b/gfx/public/nsThemeConstants.h
index 5d7faee213a..69cf2aa9f5e 100644
--- a/gfx/public/nsThemeConstants.h
+++ b/gfx/public/nsThemeConstants.h
@@ -41,6 +41,9 @@
 // The gripper for a toolbar.
 #define NS_THEME_TOOLBAR_GRIPPER                           17
 
+// A splitter.  Can be horizontal or vertical.
+#define NS_THEME_SPLITTER                                  19
+
 // A status bar in a main application window.
 #define NS_THEME_STATUSBAR                                 21