fix(theming): theming should also watch for controller changes.

As the dialog inherits the controllers not at link time, the theming won't work.
Tried a few possibilities:
- Walking through the DOM after show and re-theme the elements
- Recompile the template after show

But the most elegant and performant solution is definitely watching for the controller change.

Fixes #5899

Closes #7154

src/core/services/theming/theming.js

@@ -334,27 +334,61 @@ function ThemingProvider($mdColorPalette) {
    */
   /* @ngInject */
   function ThemingService($rootScope, $log) {
+        // Allow us to be invoked via a linking function signature.
+    var applyTheme = function (scope, el) {
+          if (el === undefined) { el = scope; scope = undefined; }
+          if (scope === undefined) { scope = $rootScope; }
+          applyTheme.inherit(el, el);
+        };
 
-    applyTheme.inherit = function(el, parent) {
-      var ctrl = parent.controller('mdTheme');
+    applyTheme.THEMES = angular.extend({}, THEMES);
+    applyTheme.inherit = inheritTheme;
+    applyTheme.registered = registered;
+    applyTheme.defaultTheme = function() { return defaultTheme; };
+    applyTheme.generateTheme = function(name) { generateTheme(name, nonce); };
+
+    return applyTheme;
 
+    /**
+     * Determine is specified theme name is a valid, registered theme
+     */
+    function registered(themeName) {
+      if (themeName === undefined || themeName === '') return true;
+      return applyTheme.THEMES[themeName] !== undefined;
+    }
+
+    /**
+     * Get theme name for the element, then update with Theme CSS class
+     */
+    function inheritTheme (el, parent) {
+      var ctrl = parent.controller('mdTheme');
       var attrThemeValue = el.attr('md-theme-watch');
-      if ( (alwaysWatchTheme || angular.isDefined(attrThemeValue)) && attrThemeValue != 'false') {
-        var deregisterWatch = $rootScope.$watch(function() {
-          return ctrl && ctrl.$mdTheme || (defaultTheme == 'default' ? '' : defaultTheme);
-        }, changeTheme);
-        el.on('$destroy', deregisterWatch);
-      } else {
-        var theme = ctrl && ctrl.$mdTheme || (defaultTheme == 'default' ? '' : defaultTheme);
-        changeTheme(theme);
+      var watchTheme = (alwaysWatchTheme || angular.isDefined(attrThemeValue)) && attrThemeValue != 'false';
+
+      updateThemeClass(lookupThemeName());
+
+      el.on('$destroy', watchTheme ? $rootScope.$watch(lookupThemeName, updateThemeClass) : angular.noop );
+
+      /**
+       * Find the theme name from the parent controller or element data
+       */
+      function lookupThemeName() {
+        // As a few components (dialog) add their controllers later, we should also watch for a controller init.
+        ctrl = parent.controller('mdTheme') || el.data('$mdThemeController');
+        return ctrl && ctrl.$mdTheme || (defaultTheme == 'default' ? '' : defaultTheme);
       }
 
-      function changeTheme(theme) {
+      /**
+       * Remove old theme class and apply a new one
+       * NOTE: if not a valid theme name, then the current name is not changed
+       */
+      function updateThemeClass(theme) {
         if (!theme) return;
         if (!registered(theme)) {
           $log.warn('Attempted to use unregistered theme \'' + theme + '\'. ' +
                     'Register it with $mdThemingProvider.theme().');
         }
+
         var oldTheme = el.data('$mdThemeName');
         if (oldTheme) el.removeClass('md-' + oldTheme +'-theme');
         el.addClass('md-' + theme + '-theme');
@@ -363,31 +397,8 @@ function ThemingProvider($mdColorPalette) {
           el.data('$mdThemeController', ctrl);
         }
       }
-    };
-
-    applyTheme.THEMES = angular.extend({}, THEMES);
-    applyTheme.defaultTheme = function() { return defaultTheme; };
-    applyTheme.registered = registered;
-    applyTheme.generateTheme = function(name) { generateTheme(name, nonce); };
-
-    return applyTheme;
-
-    function registered(themeName) {
-      if (themeName === undefined || themeName === '') return true;
-      return applyTheme.THEMES[themeName] !== undefined;
     }
 
-    function applyTheme(scope, el) {
-      // Allow us to be invoked via a linking function signature.
-      if (el === undefined) {
-        el = scope;
-        scope = undefined;
-      }
-      if (scope === undefined) {
-        scope = $rootScope;
-      }
-      applyTheme.inherit(el, el);
-    }
   }
 }