Add enable_mobile_theme site setting. Uncheck it to disable the mobile theme.

app/assets/javascripts/discourse/controllers/header_controller.js

@@ -32,6 +32,10 @@ Discourse.HeaderController = Discourse.Controller.extend({
     return Discourse.Mobile.mobileView;
   }.property(),
 
+  showMobileToggle: function() {
+    return Discourse.SiteSettings.enable_mobile_theme;
+  }.property(),
+
   toggleMobileView: function() {
     Discourse.Mobile.toggleMobileView();
   }

app/assets/javascripts/discourse/templates/header.js.handlebars

@@ -123,6 +123,7 @@
           {{#titledLinkTo "list.latest" titleKey="filters.latest.help"}}{{i18n filters.latest.title}}{{/titledLinkTo}}
         </li>
         <li>{{faqLink}}</li>
+        {{#if showMobileToggle}}
           <li>
             <a href="#" {{action toggleMobileView}}>
               {{#if mobileView}}
@@ -132,6 +133,7 @@
               {{/if}}
             </a>
           </li>
+        {{/if}}
       </ul>
 
       {{#if categories}}

app/helpers/application_helper.rb

@@ -109,6 +109,7 @@ module ApplicationHelper
   end
 
   def mobile_view?
+    return false unless SiteSetting.enable_mobile_theme
     if session[:mobile_view]
       session[:mobile_view] == '1'
     else

app/models/site_setting.rb

@@ -261,6 +261,8 @@ class SiteSetting < ActiveRecord::Base
 
   setting(:sequential_replies_threshold, 2)
 
+  client_setting(:enable_mobile_theme, true)
+
   def self.generate_api_key!
     self.api_key = SecureRandom.hex(32)
   end

config/locales/server.en.yml

@@ -696,6 +696,8 @@ en:
 
     sequential_replies_threshold: "The amount of posts a user has to make in a row in a topic before being notified"
 
+    enable_mobile_theme: "Mobile devices use a mobile-friendly theme, with the ability to switch to the full site. Disable this if you want to use a custom stylesheet that is fully responsive."
+
   notification_types:
     mentioned: "%{display_username} mentioned you in %{link}"
     liked: "%{display_username} liked your post in %{link}"

spec/helpers/application_helper_spec.rb

@@ -3,6 +3,11 @@ require 'spec_helper'
 describe ApplicationHelper do
 
   describe 'mobile_view?' do
+    context "enable_mobile_theme is true" do
+      before do
+        SiteSetting.stubs(:enable_mobile_theme).returns(true)
+      end
+
       it "is true if mobile_view is '1' in the session" do
         session[:mobile_view] = '1'
         helper.mobile_view?.should be_true
@@ -36,5 +41,34 @@ describe ApplicationHelper do
       end
     end
 
+    context "enable_mobile_theme is false" do
+      before do
+        SiteSetting.stubs(:enable_mobile_theme).returns(false)
+      end
+
+      it "is false if mobile_view is '1' in the session" do
+        session[:mobile_view] = '1'
+        helper.mobile_view?.should be_false
+      end
+
+      it "is false if mobile_view is '0' in the session" do
+        session[:mobile_view] = '0'
+        helper.mobile_view?.should be_false
+      end
+
+      context "mobile_view is not set" do
+        it "is false if user agent is not mobile" do
+          controller.request.stubs(:user_agent).returns('Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/30.0.1599.17 Safari/537.36')
+          helper.mobile_view?.should be_false
+        end
+
+        it "is false for iPhone" do
+          controller.request.stubs(:user_agent).returns('Mozilla/5.0 (iPhone; U; ru; CPU iPhone OS 4_2_1 like Mac OS X; ru) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8C148a Safari/6533.18.5')
+          helper.mobile_view?.should be_false
+        end
+      end
+    end
+  end
+
 
 end