Remove the enable_themes build flag and define.

build/config/features.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.
 
 # =============================================
 #   PLEASE DO NOT ADD MORE FLAGS TO THIS FILE
 # =============================================
 #
 # These flags are effectively global. Your feature flag should go near the
 # code it controls. Most of these items are here now because they control
@@ skipping 61 matching lines @@
 
   # Enable WebVR support by default on Android
   # Still requires command line flag to access API
   # TODO(bshe): Enable for other architecture too. Currently we only support arm
   # and arm64.
   enable_webvr = is_android && (current_cpu == "arm" || current_cpu == "arm64")
 }
 
 # Additional dependent variables -----------------------------------------------
 
-enable_themes = !is_android && !is_ios
-
 # Whether we are using the rlz library or not.  Platforms like Android send
 # rlz codes for searches but do not use the library.
 enable_rlz_support = is_win || is_mac || is_ios || is_chromeos
 enable_rlz = is_chrome_branded && enable_rlz_support
 
 # Chrome OS: whether to also build the upcoming version of
 # ChromeVox, which can then be enabled via a command-line switch.
 enable_chromevox_next = false
 #
 # =============================================
 #   PLEASE DO NOT ADD MORE FLAGS TO THIS FILE
 # =============================================
 #
 # See comment at the top.