Added a dark theme

I added a dark theme in order to solve #328.
This may fix #328 at least partially.

js/views/inbox_view.js

@@ -97,6 +97,7 @@
         applyTheme: function() {
             var theme = storage.get('theme-setting') || 'android';
             this.$el.removeClass('ios')
+                    .removeClass('android-dark')
                     .removeClass('android')
                     .addClass(theme);
         },

js/views/settings_view.js

@@ -84,6 +84,7 @@
               nameOnly: i18n('nameOnly'),
               safetyNumbersSettingDescription: i18n('safetyNumbersSettingDescription'),
               safetyNumbersSettingHeader: i18n('safetyNumbersSettingHeader'),
+              themeAndroidDark: i18n('themeAndroidDark'),
             };
         }
     });