feat(theming): log a warning if core theme isn't loaded

src/lib/core/_core.scss

@@ -36,4 +36,9 @@
     $background: map-get($theme, background);
     background-color: mat-color($background, background);
   }
+
+  // Marker that is used to determine whether the user has added a theme to their page.
+  .mat-theme-loaded-marker {
+    display: none;
+  }
 }

src/lib/core/compatibility/compatibility.ts

@@ -9,6 +9,8 @@ import {
 } from '@angular/core';
 import {DOCUMENT} from '@angular/platform-browser';
 
+/** Whether we've done the global sanity checks (e.g. a theme is loaded, there is a doctype). */
+let hasDoneGlobalChecks = false;
 
 export const MATERIAL_COMPATIBILITY_MODE = new OpaqueToken('md-compatibility-mode');
 
@@ -170,14 +172,41 @@ export class CompatibilityModule {
     };
   }
 
-  constructor(@Optional() @Inject(DOCUMENT) document: any) {
-    if (isDevMode() && typeof document && !document.doctype) {
+  constructor(@Optional() @Inject(DOCUMENT) private _document: any) {
+    if (!hasDoneGlobalChecks && isDevMode()) {
+      this._checkDoctype();
+      this._checkTheme();
+      hasDoneGlobalChecks = true;
+    }
+  }
+
+  private _checkDoctype(): void {
+    if (this._document && !this._document.doctype) {
       console.warn(
         'Current document does not have a doctype. This may cause ' +
         'some Angular Material components not to behave as expected.'
       );
     }
   }
+
+  private _checkTheme(): void {
+    if (this._document) {
+      const testElement = this._document.createElement('div');
+
+      testElement.classList.add('mat-theme-loaded-marker');
+      this._document.body.appendChild(testElement);
+
+      if (getComputedStyle(testElement).display !== 'none') {
+        console.warn(
+          'Could not find Angular Material core theme. Most Material ' +
+          'components may not work as expected. For more info refer ' +
+          'to the theming guide: https://material.angular.io/guide/theming'
+        );
+      }
+
+      this._document.body.removeChild(testElement);
+    }
+  }
 }