Remove the enable_themes build flag and define.

build/config/BUILD.gn

 # Copyright (c) 2013 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.
 
 import("//build/config/allocator.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/features.gni")
@@ skipping 118 matching lines @@
   }
   if (is_ubsan || is_ubsan_null || is_ubsan_vptr || is_ubsan_security) {
     defines += [ "UNDEFINED_SANITIZER" ]
   }
   if (use_external_popup_menu) {
     defines += [ "USE_EXTERNAL_POPUP_MENU=1" ]
   }
   if (!enable_nacl) {
     defines += [ "DISABLE_NACL" ]
   }
-  if (enable_themes) {
-    defines += [ "ENABLE_THEMES=1" ]
-  }
   if (enable_rlz) {
     defines += [ "ENABLE_RLZ" ]
   }
   if (enable_wayland_server) {
     defines += [ "ENABLE_WAYLAND_SERVER=1" ]
   }
   if (proprietary_codecs) {
     defines += [ "USE_PROPRIETARY_CODECS" ]
   }
   if (safe_browsing_mode == 1) {
@@ skipping 250 matching lines @@
       #
       # TODO(sof): determine VS2015 status and retire the setting from all
       # precompiled configurations.
       cflags_c = [ "/wd4206" ]
     } else if (is_mac) {
       precompiled_header = "build/precompile.h"
       precompiled_source = "//build/precompile.h"
     }
   }
 }