Remove the enable_themes build flag and define.

tools/grit/grit_rule.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Instantiate grit. This will produce a script target to run grit, and a
 # static library that compiles the .cc files.
 #
 # Parameters
 #
 #   source (required)
@@ skipping 222 matching lines @@
     ]
   }
 }
 
 if (enable_media_router) {
   grit_defines += [
     "-D",
     "enable_media_router",
   ]
 }
-if (enable_themes) {
-  grit_defines += [
-    "-D",
-    "enable_themes",
-  ]
-}
 
 # TODO(aberent): Enable for other platforms once the build machines have
 #                Java on them (and hence can run the closure compiler).
 _strip_resource_files = is_android
 _js_minifier = "//third_party/closure_compiler/js_minify.py"
 
 grit_resource_id_file = "//tools/gritsettings/resource_ids"
 grit_info_script = "//tools/grit/grit_info.py"
 
 template("grit") {
@@ skipping 219 matching lines @@
     if (defined(invoker.configs)) {
       configs += invoker.configs
     }
 
     if (defined(invoker.visibility)) {
       visibility = invoker.visibility
     }
     output_name = grit_output_name
   }
 }