diff --git a/app/assets/javascripts/discourse/controllers/preferences/interface.js.es6 b/app/assets/javascripts/discourse/controllers/preferences/interface.js.es6
index c52e4c5e839..4c5d920fbac 100644
--- a/app/assets/javascripts/discourse/controllers/preferences/interface.js.es6
+++ b/app/assets/javascripts/discourse/controllers/preferences/interface.js.es6
@@ -1,21 +1,30 @@
 import PreferencesTabController from "discourse/mixins/preferences-tab-controller";
 import { default as computed, observes } from "ember-addons/ember-computed-decorators";
-import { listThemes, previewTheme } from 'discourse/lib/theme-selector';
+import { listThemes, previewTheme, setLocalTheme } from 'discourse/lib/theme-selector';
 import { popupAjaxError } from 'discourse/lib/ajax-error';
 
 export default Ember.Controller.extend(PreferencesTabController, {
 
-  saveAttrNames: [
-    'locale',
-    'external_links_in_new_tab',
-    'dynamic_favicon',
-    'enable_quoting',
-    'disable_jump_reply',
-    'automatically_unpin_topics',
-    'theme_key'
-  ],
+  @computed("makeThemeDefault")
+  saveAttrNames(makeDefault) {
+    let attrs = [
+      'locale',
+      'external_links_in_new_tab',
+      'dynamic_favicon',
+      'enable_quoting',
+      'disable_jump_reply',
+      'automatically_unpin_topics'
+    ];
+
+    if (makeDefault) {
+      attrs.push('theme_key');
+    }
+
+    return attrs;
+  },
 
   preferencesController: Ember.inject.controller('preferences'),
+  makeThemeDefault: true,
 
   @computed()
   availableLocales() {
@@ -40,8 +49,15 @@ export default Ember.Controller.extend(PreferencesTabController, {
   actions: {
     save() {
       this.set('saved', false);
+      const makeThemeDefault = this.get("makeThemeDefault");
+
       return this.get('model').save(this.get('saveAttrNames')).then(() => {
         this.set('saved', true);
+
+        if (!makeThemeDefault) {
+          setLocalTheme(this.get('model.user_option.theme_key'), this.get('model.user_option.theme_key_seq'));
+        }
+
       }).catch(popupAjaxError);
     }
   }
diff --git a/app/assets/javascripts/discourse/lib/theme-selector.js.es6 b/app/assets/javascripts/discourse/lib/theme-selector.js.es6
index af0f8da3903..463e0189bbb 100644
--- a/app/assets/javascripts/discourse/lib/theme-selector.js.es6
+++ b/app/assets/javascripts/discourse/lib/theme-selector.js.es6
@@ -13,6 +13,14 @@ export function currentThemeKey() {
   return themeKey;
 }
 
+export function setLocalTheme(key, themeSeq) {
+  if (key) {
+    $.cookie('theme_key', `${key},${themeSeq}`, {path: '/', expires: 9999});
+  } else {
+    $.cookie('theme_key', null, {path: '/', expires: 1});
+  }
+}
+
 export function refreshCSS(node, hash, newHref, options) {
 
   let $orig = $(node);
diff --git a/app/assets/javascripts/discourse/templates/preferences/interface.hbs b/app/assets/javascripts/discourse/templates/preferences/interface.hbs
index b8266e906bf..ba9e1a90d59 100644
--- a/app/assets/javascripts/discourse/templates/preferences/interface.hbs
+++ b/app/assets/javascripts/discourse/templates/preferences/interface.hbs
@@ -4,6 +4,9 @@
   <div class="controls">
     {{combo-box content=userSelectableThemes value=model.user_option.theme_key}}
   </div>
+  <div class="controls">
+    {{preference-checkbox labelKey="user.theme_default_on_all_devices" checked=makeThemeDefault}}
+  </div>
 </div>
 {{/if}}
 
diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 0cd39cd3a57..f1e12606f13 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -267,13 +267,19 @@ class ApplicationController < ActionController::Base
     resolve_safe_mode
     return if request.env[NO_CUSTOM]
 
-    theme_key = flash[:preview_theme_key] || current_user&.user_option&.theme_key
+    theme_key = flash[:preview_theme_key]
 
-    # TODO 2018: delete this, old cookie cleanup code
-    if cookies[:theme_key]
-      cookies.delete(:theme_key)
+    user_option = current_user&.user_option
+
+    unless theme_key
+      key, seq = cookies[:theme_key]&.split(",")
+      if key && seq && seq.to_i == user_option&.theme_key_seq
+        theme_key = key
+      end
     end
 
+    theme_key ||= user_option&.theme_key
+
     if theme_key && !guardian.allow_theme?(theme_key)
       theme_key = nil
     end
diff --git a/app/models/user_option.rb b/app/models/user_option.rb
index 62480aca83a..551380335dd 100644
--- a/app/models/user_option.rb
+++ b/app/models/user_option.rb
@@ -158,6 +158,7 @@ end
 #  include_tl0_in_digests           :boolean          default(FALSE)
 #  notification_level_when_replying :integer
 #  theme_key                        :string
+#  theme_key_seq                    :integer          default(0), not null
 #
 # Indexes
 #
diff --git a/app/serializers/user_option_serializer.rb b/app/serializers/user_option_serializer.rb
index 534913323b6..a6a7caeb236 100644
--- a/app/serializers/user_option_serializer.rb
+++ b/app/serializers/user_option_serializer.rb
@@ -19,7 +19,8 @@ class UserOptionSerializer < ApplicationSerializer
              :email_in_reply_to,
              :like_notification_frequency,
              :include_tl0_in_digests,
-             :theme_key
+             :theme_key,
+             :theme_key_seq
 
 
   def auto_track_topics_after_msecs
diff --git a/app/services/user_updater.rb b/app/services/user_updater.rb
index 30bed45d60a..758c019c2a0 100644
--- a/app/services/user_updater.rb
+++ b/app/services/user_updater.rb
@@ -76,6 +76,11 @@ class UserUpdater
 
     save_options = false
 
+    # special handling for theme_key cause we need to bump a sequence number
+    if attributes.key?(:theme_key) && user.user_option.theme_key != attributes[:theme_key]
+      user.user_option.theme_key_seq += 1
+    end
+
     OPTION_ATTR.each do |attribute|
       if attributes.key?(attribute)
         save_options = true
diff --git a/config/locales/client.en.yml b/config/locales/client.en.yml
index 92c2a981f59..6228db50610 100644
--- a/config/locales/client.en.yml
+++ b/config/locales/client.en.yml
@@ -581,6 +581,7 @@ en:
       first_notification: "Your first notification! Select it to begin."
       disable_jump_reply: "Don't jump to my post after I reply"
       dynamic_favicon: "Show new / updated topic count on browser icon"
+      theme_default_on_all_devices: "Make this my default theme on all my devices"
       external_links_in_new_tab: "Open all external links in a new tab"
       enable_quoting: "Enable quote reply for highlighted text"
       change: "change"
diff --git a/db/migrate/20170515152725_add_theme_key_seq_to_user_options.rb b/db/migrate/20170515152725_add_theme_key_seq_to_user_options.rb
new file mode 100644
index 00000000000..7bba1cf9c55
--- /dev/null
+++ b/db/migrate/20170515152725_add_theme_key_seq_to_user_options.rb
@@ -0,0 +1,5 @@
+class AddThemeKeySeqToUserOptions < ActiveRecord::Migration
+  def change
+    add_column :user_options, :theme_key_seq, :integer, null: false, default: 0
+  end
+end
diff --git a/spec/controllers/application_controller_spec.rb b/spec/controllers/application_controller_spec.rb
index c59fb9120ca..dc86db91228 100644
--- a/spec/controllers/application_controller_spec.rb
+++ b/spec/controllers/application_controller_spec.rb
@@ -18,8 +18,15 @@ describe TopicsController do
   end
 
   describe "themes" do
+    let :theme do
+      Theme.create!(user_id: -1, name: 'bob', user_selectable: true)
+    end
+
+    let :theme2 do
+      Theme.create!(user_id: -1, name: 'bobbob', user_selectable: true)
+    end
+
     it "selects the theme the user has selected" do
-      theme = Theme.create!(user_id: -1, name: 'bob', user_selectable: true)
       user = log_in
       user.user_option.update_columns(theme_key: theme.key)
 
@@ -31,6 +38,26 @@ describe TopicsController do
       get :show, id: 666
       expect(controller.theme_key).not_to eq(theme.key)
     end
+
+    it "can be overridden with a cookie" do
+      user = log_in
+      user.user_option.update_columns(theme_key: theme.key)
+
+      cookies['theme_key'] = "#{theme2.key},#{user.user_option.theme_key_seq}"
+
+      get :show, id: 666
+      expect(controller.theme_key).to eq(theme2.key)
+
+    end
+
+    it "cookie can fail back to user if out of sync" do
+      user = log_in
+      user.user_option.update_columns(theme_key: theme.key)
+      cookies['theme_key'] = "#{theme2.key},#{user.user_option.theme_key_seq-1}"
+
+      get :show, id: 666
+      expect(controller.theme_key).to eq(theme.key)
+    end
   end
 
   it "doesn't store an incoming link when there's no referer" do
diff --git a/spec/services/user_updater_spec.rb b/spec/services/user_updater_spec.rb
index 3305f67842d..0d025c8cc41 100644
--- a/spec/services/user_updater_spec.rb
+++ b/spec/services/user_updater_spec.rb
@@ -66,6 +66,10 @@ describe UserUpdater do
       updater = UserUpdater.new(acting_user, user)
       date_of_birth = Time.zone.now
 
+      theme = Theme.create!(user_id: -1, name: "test", user_selectable: true)
+
+      seq = user.user_option.theme_key_seq
+
       val = updater.update(bio_raw: 'my new bio',
                      email_always: 'true',
                      mailing_list_mode: true,
@@ -74,7 +78,8 @@ describe UserUpdater do
                      auto_track_topics_after_msecs: 101,
                      notification_level_when_replying: 3,
                      email_in_reply_to: false,
-                     date_of_birth: date_of_birth
+                     date_of_birth: date_of_birth,
+                     theme_key: theme.key
                     )
       expect(val).to be_truthy
 
@@ -88,6 +93,8 @@ describe UserUpdater do
       expect(user.user_option.auto_track_topics_after_msecs).to eq 101
       expect(user.user_option.notification_level_when_replying).to eq 3
       expect(user.user_option.email_in_reply_to).to eq false
+      expect(user.user_option.theme_key).to eq theme.key
+      expect(user.user_option.theme_key_seq).to eq(seq+1)
       expect(user.date_of_birth).to eq(date_of_birth.to_date)
     end