feat(overlay): allow theming overlay-based components (#2967)

Fixes #2662.

guides/theming.md

@@ -44,6 +44,24 @@ Finally, if your app's content **is not** placed inside of a `md-sidenav-contain
 need to add the `mat-app-background` class to your wrapper element (for example the `body`). This
 ensures that the proper theme background is applied to your page.
 
+#### Theming overlay-based components
+Since certain components (e.g. `dialog`) are inside of a global overlay container, your theme may
+not be applied to them. In order to define the theme that will be used for overlay components, you
+have to specify it on the global `OverlayContainer` instance:
+
+```ts
+import {OverlayContainer} from '@angular/material';
+
+@NgModule({
+  // misc config goes here
+})
+export class YourAppModule {
+  constructor(overlayContainer: OverlayContainer) {
+    overlayContainer.themeClass = 'your-theme';
+  }
+}
+```
+
 ### Defining a custom theme
 When you want more customization than a pre-built theme offers, you can create your own theme file.
 

src/lib/core/overlay/overlay-container.ts

@@ -9,6 +9,24 @@ import {Injectable, Optional, SkipSelf} from '@angular/core';
 export class OverlayContainer {
   protected _containerElement: HTMLElement;
 
+  private _themeClass: string;
+
+  /**
+   * Base theme to be applied to all overlay-based components.
+   */
+  get themeClass(): string { return this._themeClass; }
+  set themeClass(value: string) {
+    if (this._containerElement) {
+      this._containerElement.classList.remove(this._themeClass);
+
+      if (value) {
+        this._containerElement.classList.add(value);
+      }
+    }
+
+    this._themeClass = value;
+  }
+
   /**
    * This method returns the overlay container element.  It will lazily
    * create the element the first time  it is called to facilitate using
@@ -27,6 +45,11 @@ export class OverlayContainer {
   protected _createContainer(): void {
     let container = document.createElement('div');
     container.classList.add('cdk-overlay-container');
+
+    if (this._themeClass) {
+      container.classList.add(this._themeClass);
+    }
+
     document.body.appendChild(container);
     this._containerElement = container;
   }

src/lib/core/overlay/overlay.spec.ts

@@ -272,6 +272,34 @@ describe('Overlay', () => {
   });
 });
 
+describe('OverlayContainer theming', () => {
+  let overlayContainer: OverlayContainer;
+  let overlayContainerElement: HTMLElement;
+
+  beforeEach(async(() => {
+    TestBed.configureTestingModule({ imports: [OverlayContainerThemingTestModule] });
+    TestBed.compileComponents();
+  }));
+
+  beforeEach(inject([OverlayContainer], (o: OverlayContainer) => {
+    overlayContainer = o;
+    overlayContainerElement = overlayContainer.getContainerElement();
+  }));
+
+  it('should be able to set a theme on the overlay container', () => {
+    overlayContainer.themeClass = 'my-theme';
+    expect(overlayContainerElement.classList).toContain('my-theme');
+  });
+
+  it('should clear any previously-set themes when a new theme is set', () => {
+    overlayContainer.themeClass = 'initial-theme';
+    expect(overlayContainerElement.classList).toContain('initial-theme');
+
+    overlayContainer.themeClass = 'new-theme';
+    expect(overlayContainerElement.classList).not.toContain('initial-theme');
+    expect(overlayContainerElement.classList).toContain('new-theme');
+  });
+});
 
 /** Simple component for testing ComponentPortal. */
 @Component({template: '<p>Pizza</p>'})
@@ -297,6 +325,12 @@ const TEST_COMPONENTS = [PizzaMsg, TestComponentWithTemplatePortals];
 })
 class OverlayTestModule { }
 
+/** Component for testing the overlay container theming. */
+@NgModule({
+  imports: [OverlayModule, PortalModule],
+})
+class OverlayContainerThemingTestModule { }
+
 class FakePositionStrategy implements PositionStrategy {
   apply(element: Element): Promise<void> {
     element.classList.add('fake-positioned');