diff -r 10efb1797e7b Lib/idlelib/configDialog.py
--- a/Lib/idlelib/configDialog.py	Thu Oct 01 13:16:43 2015 +0200
+++ b/Lib/idlelib/configDialog.py	Sun Oct 04 12:43:46 2015 -0700
@@ -1113,6 +1113,18 @@
             if idleConf.defaultCfg[configType].Get(section, item) == value:
                 #the setting equals a default setting, remove it from user cfg
                 return idleConf.userCfg[configType].RemoveOption(section, item)
+        # handle any special cases and backwards compatiblity concerns
+        if configType == 'main' and section == 'Theme' and item == 'name':
+            # For backwards compatibility, we write 'newer' themes to the
+            # user config file so they can be used in older versions of IDLE.
+            # These are ignored by newer versions which have the theme name
+            # in their default list. See Issue #25313
+            if value in idleConf.defaultCfg['highlight'].sections() \
+                    and value not in ['IDLE Classic', 'IDLE New'] \
+                    and value not in idleConf.userCfg['highlight'].sections():
+                for elt,val in idleConf.defaultCfg['highlight'].items(value):
+                    idleConf.userCfg['highlight'].SetOption(value, elt, val)
+                idleConf.userCfg['highlight'].Save()
         #if we got here set the option
         return idleConf.userCfg[configType].SetOption(section, item, value)
 
diff -r 10efb1797e7b Lib/idlelib/configHandler.py
--- a/Lib/idlelib/configHandler.py	Thu Oct 01 13:16:43 2015 +0200
+++ b/Lib/idlelib/configHandler.py	Sun Oct 04 12:43:46 2015 -0700
@@ -270,13 +270,14 @@
         """
         if not (configType in self.config_types):
             raise InvalidConfigType('Invalid configType specified')
-        if configSet == 'user':
-            cfgParser = self.userCfg[configType]
-        elif configSet == 'default':
-            cfgParser=self.defaultCfg[configType]
+        if not (configSet in ['user', 'default']):
+            raise InvalidConfigSet('Invalid configSet specified')
+        defaultsections = self.defaultCfg[configType].sections()
+        if configSet == 'default':
+            return defaultsections
         else:
-            raise InvalidConfigSet('Invalid configSet specified')
-        return cfgParser.sections()
+            usersections = self.userCfg[configType].sections()
+            return [x for x in usersections if x not in defaultsections]
 
     def GetHighlight(self, theme, element, fgBg=None):
         """Return individual theme element highlight color(s).