Remove the enable_themes build flag and define.

chrome/browser/ui/BUILD.gn

 # 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.
 
 import("//build/config/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/split_static_library.gni")
 import("//chrome/common/features.gni")
@@ skipping 2083 matching lines @@
       "webui/settings/certificates_handler.cc",
       "webui/settings/certificates_handler.h",
     ]
   }
   if (is_mac || is_win) {
     sources += [
       "webui/settings/native_certificates_handler.cc",
       "webui/settings/native_certificates_handler.h",
     ]
   }
-  if (enable_themes) {
-    sources += [
-      "webui/theme_source.cc",
-      "webui/theme_source.h",
-    ]
-    deps += [ "//chrome/browser:theme_properties" ]
-  }
   if (enable_print_preview) {
     sources += [
       "webui/print_preview/extension_printer_handler.cc",
       "webui/print_preview/extension_printer_handler.h",
       "webui/print_preview/print_preview_handler.cc",
       "webui/print_preview/print_preview_handler.h",
       "webui/print_preview/print_preview_ui.cc",
       "webui/print_preview/print_preview_ui.h",
       "webui/print_preview/printer_backend_proxy.h",
       "webui/print_preview/printer_capabilities.cc",
@@ skipping 146 matching lines @@
         assert(enable_vr_shell)
         defines += [ "ENABLE_VR_SHELL_UI_DEV" ]
       }
       deps += [
         "//chrome/browser:jni_headers",
         "//components/web_contents_delegate_android",
       ]
       deps -= [ "//ui/events" ]
     }
   } else {
-    sources += [ "browser_window.h" ]
+    # !is_android
+    sources += [
+      "browser_window.h",
+      "webui/theme_source.cc",
+      "webui/theme_source.h",
+    ]
+    deps += [ "//chrome/browser:theme_properties" ]
   }
 
   if (is_mac) {
     sources += [
       "certificate_viewer_mac.h",
       "certificate_viewer_mac.mm",
       "cocoa/accelerator_utils_cocoa.mm",
       "cocoa/accelerators_cocoa.h",
       "cocoa/accelerators_cocoa.mm",
       "cocoa/apps/app_shim_menu_controller_mac.h",
@@ skipping 1194 matching lines @@
       "test/test_confirm_bubble_model.cc",
       "test/test_confirm_bubble_model.h",
     ]
     deps += [ "//chrome/test:test_support_ui" ]
   }
 
   if (enable_extensions) {
     deps += [ "//extensions/browser" ]
   }
 }