diff -r 193327cabbbc Lib/idlelib/ClassBrowser.py
--- a/Lib/idlelib/ClassBrowser.py	Mon Nov 02 09:15:47 2015 -0800
+++ b/Lib/idlelib/ClassBrowser.py	Wed Nov 04 04:15:42 2015 -0500
@@ -56,7 +56,7 @@
         self.settitle()
         top.focus_set()
         # create scrolled canvas
-        theme = idleConf.GetOption('main','Theme','name')
+        theme = idleConf.CurrentTheme()
         background = idleConf.GetHighlight(theme, 'normal')['background']
         sc = ScrolledCanvas(top, bg=background, highlightthickness=0, takefocus=1)
         sc.frame.pack(expand=1, fill="both")
diff -r 193327cabbbc Lib/idlelib/ColorDelegator.py
--- a/Lib/idlelib/ColorDelegator.py	Mon Nov 02 09:15:47 2015 -0800
+++ b/Lib/idlelib/ColorDelegator.py	Wed Nov 04 04:15:42 2015 -0500
@@ -60,7 +60,7 @@
         self.tag_raise('sel')
 
     def LoadTagDefs(self):
-        theme = idleConf.GetOption('main','Theme','name')
+        theme = idleConf.CurrentTheme()
         self.tagdefs = {
             "COMMENT": idleConf.GetHighlight(theme, "comment"),
             "KEYWORD": idleConf.GetHighlight(theme, "keyword"),
diff -r 193327cabbbc Lib/idlelib/EditorWindow.py
--- a/Lib/idlelib/EditorWindow.py	Mon Nov 02 09:15:47 2015 -0800
+++ b/Lib/idlelib/EditorWindow.py	Wed Nov 04 04:15:42 2015 -0500
@@ -739,7 +739,7 @@
         # Called from self.filename_change_hook and from configDialog.py
         self._rmcolorizer()
         self._addcolorizer()
-        theme = idleConf.GetOption('main','Theme','name')
+        theme = idleConf.CurrentTheme()
         normal_colors = idleConf.GetHighlight(theme, 'normal')
         cursor_color = idleConf.GetHighlight(theme, 'cursor', fgBg='fg')
         select_colors = idleConf.GetHighlight(theme, 'hilite')
diff -r 193327cabbbc Lib/idlelib/PyShell.py
--- a/Lib/idlelib/PyShell.py	Mon Nov 02 09:15:47 2015 -0800
+++ b/Lib/idlelib/PyShell.py	Wed Nov 04 04:15:42 2015 -0500
@@ -152,7 +152,7 @@
             # possible due to update in restore_file_breaks
             return
         if color:
-            theme = idleConf.GetOption('main','Theme','name')
+            theme = idleConf.CurrentTheme()
             cfg = idleConf.GetHighlight(theme, "break")
         else:
             cfg = {'foreground': '', 'background': ''}
@@ -338,7 +338,7 @@
 
     def LoadTagDefs(self):
         ColorDelegator.LoadTagDefs(self)
-        theme = idleConf.GetOption('main','Theme','name')
+        theme = idleConf.CurrentTheme()
         self.tagdefs.update({
             "stdin": {'background':None,'foreground':None},
             "stdout": idleConf.GetHighlight(theme, "stdout"),
@@ -621,7 +621,7 @@
         item = RemoteObjectBrowser.StubObjectTreeItem(self.rpcclt, oid)
         from idlelib.TreeWidget import ScrolledCanvas, TreeNode
         top = Toplevel(self.tkconsole.root)
-        theme = idleConf.GetOption('main','Theme','name')
+        theme = idleConf.CurrentTheme()
         background = idleConf.GetHighlight(theme, 'normal')['background']
         sc = ScrolledCanvas(top, bg=background, highlightthickness=0)
         sc.frame.pack(expand=1, fill="both")
diff -r 193327cabbbc Lib/idlelib/TreeWidget.py
--- a/Lib/idlelib/TreeWidget.py	Mon Nov 02 09:15:47 2015 -0800
+++ b/Lib/idlelib/TreeWidget.py	Wed Nov 04 04:15:42 2015 -0500
@@ -249,7 +249,7 @@
         except AttributeError:
             # padding carefully selected (on Windows) to match Entry widget:
             self.label = Label(self.canvas, text=text, bd=0, padx=2, pady=2)
-        theme = idleConf.GetOption('main','Theme','name')
+        theme = idleConf.CurrentTheme()
         if self.selected:
             self.label.configure(idleConf.GetHighlight(theme, 'hilite'))
         else:
diff -r 193327cabbbc Lib/idlelib/config-main.def
--- a/Lib/idlelib/config-main.def	Mon Nov 02 09:15:47 2015 -0800
+++ b/Lib/idlelib/config-main.def	Wed Nov 04 04:15:42 2015 -0500
@@ -65,6 +65,8 @@
 [Theme]
 default= 1
 name= IDLE Classic
+name2=
+# name2 set in user main.cfg for default themes added after 2015 Oct 1
 
 [Keys]
 default= 1
diff -r 193327cabbbc Lib/idlelib/configDialog.py
--- a/Lib/idlelib/configDialog.py	Mon Nov 02 09:15:47 2015 -0800
+++ b/Lib/idlelib/configDialog.py	Wed Nov 04 04:15:42 2015 -0500
@@ -263,6 +263,7 @@
         self.buttonDeleteCustomTheme=Button(
                 frameTheme, text='Delete Custom Theme',
                 command=self.DeleteCustomTheme)
+        self.new_custom_theme = Label(frameTheme, bd=2)
 
         ##widget packing
         #body
@@ -286,6 +287,7 @@
         self.optMenuThemeBuiltin.pack(side=TOP, fill=X, padx=5, pady=5)
         self.optMenuThemeCustom.pack(side=TOP, fill=X, anchor=W, padx=5, pady=5)
         self.buttonDeleteCustomTheme.pack(side=TOP, fill=X, padx=5, pady=5)
+        self.new_custom_theme.pack(side=TOP, fill=X, pady=5)
         return frame
 
     def CreatePageKeys(self):
@@ -503,20 +505,14 @@
     def VarChanged_builtinTheme(self, *params):
         value = self.builtinTheme.get()
         if value == 'IDLE Dark':
-            tkMessageBox.showwarning(
-                title="The 'IDLE Dark' Text Color Theme",
-                message="IDLE Dark is new in October, 2015.  Trying to "
-                        "run earlier versions of IDLE with it selected "
-                        "will disable colorizing, or worse.\n\n"
-                        "If you might ever run an earlier release of IDLE, "
-                        "then before exiting this version, "
-                        "either switch to another theme or "
-                        "hit the 'Save as New Custom Theme' button.  "
-                        "The latter requires a new name, such as "
-                        "'Custom Dark', but the custom theme will work "
-                        "with any IDLE release, and can be modified.",
-                parent=self)
-        self.AddChangedItem('main', 'Theme', 'name', value)
+            self.AddChangedItem('main', 'Theme', 'name', 'IDLE Classic')
+            self.AddChangedItem('main', 'Theme', 'name2', value)
+            self.new_custom_theme.config(text='New theme, see Help',
+                                         fg='#500000')
+        else:
+            self.AddChangedItem('main', 'Theme', 'name', value)
+            self.AddChangedItem('main', 'Theme', 'name2', '')
+            self.new_custom_theme.config(text='', fg='black')
         self.PaintThemeSample()
 
     def VarChanged_customTheme(self, *params):
@@ -1350,14 +1346,14 @@
 When you click either the Apply or Ok buttons, settings in this
 dialog that are different from IDLE's default are saved in
 a .idlerc directory in your home directory. Except as noted,
-hese changes apply to all versions of IDLE installed on this
+these changes apply to all versions of IDLE installed on this
 machine. Some do not take affect until IDLE is restarted.
 [Cancel] only cancels changes made since the last save.
 '''
 help_pages = {
     'Highlighting':'''
 Highlighting:
-The IDLE Dark color theme is new in Octover 2015.  It can only
+The IDLE Dark color theme is new in October 2015.  It can only
 be used with older IDLE releases if it is saved as a custom
 theme, with a different name.
 '''
diff -r 193327cabbbc Lib/idlelib/configHandler.py
--- a/Lib/idlelib/configHandler.py	Mon Nov 02 09:15:47 2015 -0800
+++ b/Lib/idlelib/configHandler.py	Wed Nov 04 04:15:42 2015 -0500
@@ -373,7 +373,16 @@
 
     def CurrentTheme(self):
         "Return the name of the currently active theme."
-        return self.GetOption('main', 'Theme', 'name', default='')
+        default = self.GetOption('main', 'Theme', 'default', default=True)
+        if default:
+            theme = self.GetOption('main', 'Theme', 'name2', default='')
+        if default and not theme or not default:
+            theme = self.GetOption('main', 'Theme', 'name', default='')
+        source = self.defaultCfg if default else self.userCfg
+        if source['highlight'].has_section(theme):
+                return theme
+        else:    
+            return "IDLE Classic"
 
     def CurrentKeys(self):
         "Return the name of the currently active key set."