diff --git a/.changeset/nervous-hounds-sleep.md b/.changeset/nervous-hounds-sleep.md
new file mode 100644
index 0000000000..530ff348f7
--- /dev/null
+++ b/.changeset/nervous-hounds-sleep.md
@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-app-api': patch
+---
+
+Make themes configurable through extensions, and switched default themes to use extensions instead.
diff --git a/.changeset/silly-swans-invent.md b/.changeset/silly-swans-invent.md
new file mode 100644
index 0000000000..68b094aba7
--- /dev/null
+++ b/.changeset/silly-swans-invent.md
@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-plugin-api': patch
+---
+
+Added `createThemeExtension` and `coreExtensionData.theme`.
diff --git a/packages/frontend-app-api/package.json b/packages/frontend-app-api/package.json
index 0982cb0157..6c45e9239a 100644
--- a/packages/frontend-app-api/package.json
+++ b/packages/frontend-app-api/package.json
@@ -25,7 +25,8 @@
   "devDependencies": {
     "@backstage/cli": "workspace:^",
     "@backstage/test-utils": "workspace:^",
-    "@testing-library/jest-dom": "^5.10.1"
+    "@testing-library/jest-dom": "^5.10.1",
+    "@testing-library/react": "^12.1.3"
   },
   "configSchema": "config.d.ts",
   "files": [
@@ -39,8 +40,10 @@
     "@backstage/core-plugin-api": "workspace:^",
     "@backstage/frontend-plugin-api": "workspace:^",
     "@backstage/plugin-graphiql": "workspace:^",
+    "@backstage/theme": "workspace:^",
     "@backstage/types": "workspace:^",
     "@material-ui/core": "^4.12.4",
+    "@material-ui/icons": "^4.11.3",
     "@types/react": "^16.13.1 || ^17.0.0",
     "lodash": "^4.17.21"
   },
diff --git a/packages/frontend-app-api/src/extensions/themes.tsx b/packages/frontend-app-api/src/extensions/themes.tsx
new file mode 100644
index 0000000000..ea11b6a638
--- /dev/null
+++ b/packages/frontend-app-api/src/extensions/themes.tsx
@@ -0,0 +1,44 @@
+/*
+ * Copyright 2023 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import React from 'react';
+import {
+  UnifiedThemeProvider,
+  themes as builtinThemes,
+} from '@backstage/theme';
+import DarkIcon from '@material-ui/icons/Brightness2';
+import LightIcon from '@material-ui/icons/WbSunny';
+import { createThemeExtension } from '@backstage/frontend-plugin-api';
+
+export const LightTheme = createThemeExtension({
+  id: 'light',
+  title: 'Light Theme',
+  variant: 'light',
+  icon: <LightIcon />,
+  Provider: ({ children }) => (
+    <UnifiedThemeProvider theme={builtinThemes.light} children={children} />
+  ),
+});
+
+export const DarkTheme = createThemeExtension({
+  id: 'dark',
+  title: 'Dark Theme',
+  variant: 'dark',
+  icon: <DarkIcon />,
+  Provider: ({ children }) => (
+    <UnifiedThemeProvider theme={builtinThemes.dark} children={children} />
+  ),
+});
diff --git a/packages/frontend-app-api/src/wiring/createApp.test.tsx b/packages/frontend-app-api/src/wiring/createApp.test.tsx
index 903b044ed3..f6172f5a4b 100644
--- a/packages/frontend-app-api/src/wiring/createApp.test.tsx
+++ b/packages/frontend-app-api/src/wiring/createApp.test.tsx
@@ -18,10 +18,11 @@ import {
   createExtension,
   createPageExtension,
   createPlugin,
+  createThemeExtension,
 } from '@backstage/frontend-plugin-api';
-import { createInstances } from './createApp';
-
-import { MockConfigApi } from '@backstage/test-utils';
+import { createApp, createInstances } from './createApp';
+import { screen } from '@testing-library/react';
+import { MockConfigApi, renderWithEffects } from '@backstage/test-utils';
 import React from 'react';
 import { createRouteRef } from '@backstage/core-plugin-api';
 
@@ -104,3 +105,33 @@ describe('createInstances', () => {
     );
   });
 });
+
+describe('createApp', () => {
+  it('should allow themes to be installed', async () => {
+    const app = createApp({
+      configLoader: async () =>
+        new MockConfigApi({
+          app: {
+            extensions: [{ 'themes.light': false }, { 'themes.dark': false }],
+          },
+        }),
+      plugins: [
+        createPlugin({
+          id: 'test',
+          extensions: [
+            createThemeExtension({
+              id: 'derp',
+              title: 'Derp',
+              variant: 'dark',
+              Provider: () => <div>Derp</div>,
+            }),
+          ],
+        }),
+      ],
+    });
+
+    await renderWithEffects(app.createRoot());
+
+    await expect(screen.findByText('Derp')).resolves.toBeInTheDocument();
+  });
+});
diff --git a/packages/frontend-app-api/src/wiring/createApp.tsx b/packages/frontend-app-api/src/wiring/createApp.tsx
index dc7db79022..deb19d919d 100644
--- a/packages/frontend-app-api/src/wiring/createApp.tsx
+++ b/packages/frontend-app-api/src/wiring/createApp.tsx
@@ -50,6 +50,7 @@ import {
   attachComponentData,
   useRouteRef,
   identityApiRef,
+  AppTheme,
 } from '@backstage/core-plugin-api';
 import { getAvailablePlugins } from './discovery';
 import {
@@ -77,10 +78,10 @@ import {
   apis as defaultApis,
   components as defaultComponents,
   icons as defaultIcons,
-  themes as defaultThemes,
 } from '../../../app-defaults/src/defaults';
 import { BrowserRouter, Route } from 'react-router-dom';
 import { SidebarItem } from '@backstage/core-components';
+import { DarkTheme, LightTheme } from '../extensions/themes';
 
 /** @public */
 export interface ExtensionTreeNode {
@@ -171,7 +172,14 @@ export function createInstances(options: {
   plugins: BackstagePlugin[];
   config: Config;
 }) {
-  const builtinExtensions = [Core, CoreRoutes, CoreNav, CoreLayout];
+  const builtinExtensions = [
+    Core,
+    CoreRoutes,
+    CoreNav,
+    CoreLayout,
+    LightTheme,
+    DarkTheme,
+  ];
 
   // pull in default extension instance from discovered packages
   // apply config to adjust default extension instances and add more
@@ -375,6 +383,12 @@ function createApiHolder(
       ?.map(e => e.getData(coreExtensionData.apiFactory))
       .filter((x): x is AnyApiFactory => !!x) ?? [];
 
+  const themeExtensions =
+    coreExtension.attachments
+      .get('themes')
+      ?.map(e => e.getData(coreExtensionData.theme))
+      .filter((x): x is AppTheme => !!x) ?? [];
+
   for (const factory of [...defaultApis, ...pluginApis]) {
     factoryRegistry.register('default', factory);
   }
@@ -422,7 +436,7 @@ function createApiHolder(
     api: appThemeApiRef,
     deps: {},
     // TODO: add extension for registering themes
-    factory: () => AppThemeSelector.createWithStorage(defaultThemes),
+    factory: () => AppThemeSelector.createWithStorage(themeExtensions),
   });
 
   factoryRegistry.register('static', {
diff --git a/packages/frontend-plugin-api/api-report.md b/packages/frontend-plugin-api/api-report.md
index 5855976bc4..e28d7d5993 100644
--- a/packages/frontend-plugin-api/api-report.md
+++ b/packages/frontend-plugin-api/api-report.md
@@ -7,6 +7,7 @@
 
 import { AnyApiFactory } from '@backstage/core-plugin-api';
 import { AnyApiRef } from '@backstage/core-plugin-api';
+import { AppTheme } from '@backstage/core-plugin-api';
 import { IconComponent } from '@backstage/core-plugin-api';
 import { JsonObject } from '@backstage/types';
 import { JSX as JSX_2 } from 'react';
@@ -71,6 +72,7 @@ export const coreExtensionData: {
   apiFactory: ConfigurableExtensionDataRef<AnyApiFactory, {}>;
   routeRef: ConfigurableExtensionDataRef<RouteRef, {}>;
   navTarget: ConfigurableExtensionDataRef<NavTarget, {}>;
+  theme: ConfigurableExtensionDataRef<AppTheme, {}>;
 };
 
 // @public (undocumented)
@@ -199,6 +201,9 @@ export function createSchemaFromZod<TOutput, TInput>(
   schemaCreator: (zImpl: typeof z) => ZodSchema<TOutput, ZodTypeDef, TInput>,
 ): PortableSchema<TOutput>;
 
+// @public (undocumented)
+export function createThemeExtension(theme: AppTheme): Extension<never>;
+
 // @public (undocumented)
 export interface Extension<TConfig> {
   // (undocumented)
diff --git a/packages/frontend-plugin-api/src/extensions/createThemeExtension.ts b/packages/frontend-plugin-api/src/extensions/createThemeExtension.ts
new file mode 100644
index 0000000000..5c13dd199e
--- /dev/null
+++ b/packages/frontend-plugin-api/src/extensions/createThemeExtension.ts
@@ -0,0 +1,32 @@
+/*
+ * Copyright 2023 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { createExtension, coreExtensionData } from '../wiring';
+import { AppTheme } from '@backstage/core-plugin-api';
+
+/** @public */
+export function createThemeExtension(theme: AppTheme) {
+  return createExtension({
+    id: `themes.${theme.id}`,
+    at: 'core/themes',
+    output: {
+      theme: coreExtensionData.theme,
+    },
+    factory({ bind }) {
+      bind({ theme });
+    },
+  });
+}
diff --git a/packages/frontend-plugin-api/src/extensions/index.ts b/packages/frontend-plugin-api/src/extensions/index.ts
index 0ec0cdf652..00cf4f919c 100644
--- a/packages/frontend-plugin-api/src/extensions/index.ts
+++ b/packages/frontend-plugin-api/src/extensions/index.ts
@@ -17,3 +17,4 @@
 export { createApiExtension } from './createApiExtension';
 export { createPageExtension } from './createPageExtension';
 export { createNavItemExtension } from './createNavItemExtension';
+export { createThemeExtension } from './createThemeExtension';
diff --git a/packages/frontend-plugin-api/src/wiring/coreExtensionData.ts b/packages/frontend-plugin-api/src/wiring/coreExtensionData.ts
index 079d5bcb18..6913501907 100644
--- a/packages/frontend-plugin-api/src/wiring/coreExtensionData.ts
+++ b/packages/frontend-plugin-api/src/wiring/coreExtensionData.ts
@@ -17,6 +17,7 @@
 import { JSX } from 'react';
 import {
   AnyApiFactory,
+  AppTheme,
   IconComponent,
   RouteRef,
 } from '@backstage/core-plugin-api';
@@ -36,4 +37,5 @@ export const coreExtensionData = {
   apiFactory: createExtensionDataRef<AnyApiFactory>('core.api.factory'),
   routeRef: createExtensionDataRef<RouteRef>('core.routing.ref'),
   navTarget: createExtensionDataRef<NavTarget>('core.nav.target'),
+  theme: createExtensionDataRef<AppTheme>('core.theme'),
 };
diff --git a/yarn.lock b/yarn.lock
index f7e19c3179..3dd3f6315b 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -4307,9 +4307,12 @@ __metadata:
     "@backstage/frontend-plugin-api": "workspace:^"
     "@backstage/plugin-graphiql": "workspace:^"
     "@backstage/test-utils": "workspace:^"
+    "@backstage/theme": "workspace:^"
     "@backstage/types": "workspace:^"
     "@material-ui/core": ^4.12.4
+    "@material-ui/icons": ^4.11.3
     "@testing-library/jest-dom": ^5.10.1
+    "@testing-library/react": ^12.1.3
     "@types/react": ^16.13.1 || ^17.0.0
     lodash: ^4.17.21
   peerDependencies: