Add support for themes (fixes #1925)

gui/default/syncthing/core/modalDirective.js

@@ -0,0 +1,16 @@
+angular.module('syncthing.core')
+    .directive('modal', function () {
+        return {
+            restrict: 'E',
+            templateUrl: 'modal.html',
+            replace: true,
+            transclude: true,
+            scope: {
+                title: '@',
+                status: '@',
+                icon: '@',
+                close: '@',
+                large: '@'
+            }
+        };
+    });