diff --git a/build-tools/tasks/generate-environment.js b/build-tools/tasks/generate-environment.js
index f43ecd1b4a..6a12bac23c 100644
--- a/build-tools/tasks/generate-environment.js
+++ b/build-tools/tasks/generate-environment.js
@@ -1,45 +1,69 @@
 // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
+
+// Generates the `internal/environment` files for each theme.
+// For every theme we write the same set of values in three forms:
+//   - environment.json  raw values
+//   - environment.js    runtime module, e.g. `export var THEME = "default";`
+//   - environment.d.ts  type declarations, e.g. `export const THEME: string;`
+
 const path = require('path');
 const { writeFile } = require('../utils/files');
 const themes = require('../utils/themes');
 const workspace = require('../utils/workspace');
 
 const ALWAYS_VISUAL_REFRESH = process.env.ALWAYS_VISUAL_REFRESH === 'true';
-const INCLUDE_ONE_THEME = process.env.INCLUDE_ONE_THEME === 'true';
 
-function writeEnvironmentFile(theme) {
-  const filepath = 'internal/environment';
-  const values = {
+// The build-time constants exposed to the component source for a given theme.
+function getEnvironmentValues(theme) {
+  return {
     PACKAGE_SOURCE: workspace.packageSource,
     PACKAGE_VERSION: workspace.packageVersion,
     GIT_SHA: workspace.gitCommitVersion,
     THEME: theme.name,
     SYSTEM: 'core',
     ALWAYS_VISUAL_REFRESH: !!theme.alwaysVisualRefresh || ALWAYS_VISUAL_REFRESH,
-    INCLUDE_ONE_THEME: INCLUDE_ONE_THEME,
+    INCLUDED_THEMES: theme.includedThemes ?? [],
   };
-  const basePath = path.join(theme.outputPath, filepath);
+}
+
+function toTypeScriptType(value) {
+  if (Array.isArray(value)) {
+    return 'string[]';
+  }
+  if (typeof value === 'boolean') {
+    return 'boolean';
+  }
+  return 'string';
+}
+
+// Source for environment.js, e.g. `export var THEME = "default";`
+function toModuleSource(values) {
+  return Object.entries(values)
+    .map(([name, value]) => `export var ${name} = ${JSON.stringify(value)};`)
+    .join('\n');
+}
+
+// Source for environment.d.ts, e.g. `export const THEME: string;`
+function toDeclarationsSource(values) {
+  return Object.entries(values)
+    .map(([name, value]) => `export const ${name}: ${toTypeScriptType(value)};`)
+    .join('\n');
+}
+
+function writeEnvironmentFile(theme) {
+  const values = getEnvironmentValues(theme);
+  const basePath = path.join(theme.outputPath, 'internal/environment');
 
   console.log('Environment values\n', JSON.stringify(values, null, 2));
 
   writeFile(`${basePath}.json`, JSON.stringify(values, null, 2));
-  writeFile(
-    `${basePath}.js`,
-    Object.entries(values)
-      .map(([key, value]) => `export var ${key} = ${JSON.stringify(value)};`)
-      .join('\n')
-  );
-  writeFile(
-    `${basePath}.d.ts`,
-    Object.keys(values)
-      .map(key => `export const ${key}: string;`)
-      .join('\n')
-  );
+  writeFile(`${basePath}.js`, toModuleSource(values));
+  writeFile(`${basePath}.d.ts`, toDeclarationsSource(values));
 }
 
 module.exports = function generateEnvironment() {
-  themes.forEach(theme => writeEnvironmentFile(theme));
+  themes.forEach(writeEnvironmentFile);
 
   return Promise.resolve();
 };
diff --git a/build-tools/utils/themes.js b/build-tools/utils/themes.js
index 6564bf32d4..0817937478 100644
--- a/build-tools/utils/themes.js
+++ b/build-tools/utils/themes.js
@@ -3,7 +3,36 @@
 const path = require('path');
 const workspace = require('./workspace');
 
-const INCLUDE_ONE_THEME = process.env.INCLUDE_ONE_THEME === 'true';
+// Secondary themes are layered on top of the primary theme as opt-in overrides. Each entry maps a
+// theme id to its style-dictionary entry point. To add a new theme, register it here, then include
+// it via the THEMES env var.
+const SECONDARY_THEME_PATHS = {
+  'visual-refresh': './visual-refresh-secondary/index.js',
+  'one-theme': './one-theme/index.js',
+};
+
+// Secondary themes included when THEMES is not set. Keeps visual refresh in, one-theme opt-in.
+const DEFAULT_THEMES = 'visual-refresh';
+
+// Resolves which secondary themes to compile from the comma-separated THEMES env var, e.g.
+// `THEMES=visual-refresh,one-theme`. Throws if a requested theme is not registered above.
+function resolveIncludedThemes() {
+  const themeIds = (process.env.THEMES ?? DEFAULT_THEMES)
+    .split(',')
+    .map(id => id.trim())
+    .filter(Boolean);
+
+  for (const id of themeIds) {
+    if (!SECONDARY_THEME_PATHS[id]) {
+      const availableThemes = Object.keys(SECONDARY_THEME_PATHS).join(', ');
+      throw new Error(`Unknown theme "${id}" in THEMES env var. Available themes: ${availableThemes}.`);
+    }
+  }
+
+  return themeIds;
+}
+
+const includedThemes = resolveIncludedThemes();
 
 const themes = [
   // This is the default Cloudscape theme, which is best used with Visual Refresh enabled (by default)
@@ -15,10 +44,8 @@ const themes = [
     designTokensPackageJson: { name: '@cloudscape-design/design-tokens' },
     outputPath: path.join(workspace.targetPath, 'components'),
     primaryThemePath: './classic/index.js',
-    secondaryThemePaths: [
-      './visual-refresh-secondary/index.js',
-      ...(INCLUDE_ONE_THEME ? ['./one-theme/index.js'] : []),
-    ],
+    includedThemes,
+    secondaryThemePaths: includedThemes.map(id => SECONDARY_THEME_PATHS[id]),
   },
 ];
 
diff --git a/pages/app/app-context.tsx b/pages/app/app-context.tsx
index 4807fdf5fe..634c9b9325 100644
--- a/pages/app/app-context.tsx
+++ b/pages/app/app-context.tsx
@@ -12,7 +12,7 @@ interface AppUrlParams {
   density: Density;
   direction: 'ltr' | 'rtl';
   visualRefresh: boolean;
-  oneTheme: boolean;
+  theme?: string;
   motionDisabled: boolean;
   appLayoutWidget: boolean;
   mode?: Mode;
@@ -33,7 +33,6 @@ const appContextDefaults: AppContextType = {
     density: Density.Comfortable,
     direction: 'ltr',
     visualRefresh: THEME === 'default',
-    oneTheme: false,
     motionDisabled: false,
     appLayoutWidget: false,
   },
diff --git a/pages/app/components/header.tsx b/pages/app/components/header.tsx
index a8ddc58445..b99df8af24 100644
--- a/pages/app/components/header.tsx
+++ b/pages/app/components/header.tsx
@@ -1,7 +1,7 @@
 // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 import React, { useContext } from 'react';
-import { Link } from 'react-router-dom';
+import { Link, useLocation } from 'react-router-dom';
 import clsx from 'clsx';
 
 import AppContext from '../app-context';
@@ -11,11 +11,13 @@ import styles from './header.scss';
 
 export default function Header({ sticky }: { sticky?: boolean }) {
   const { mode } = useContext(AppContext);
+  // Preserve the query string (theme, density, motion, …) when navigating back to the index.
+  const { search } = useLocation();
   return (
     <>
       {/* #h selector for compatibility with global navigation */}
       <header id="h" className={clsx(styles.header, sticky && styles['header-sticky'])}>
-        <Link to={`/${mode}/`}>Demo Assets</Link>
+        <Link to={{ pathname: `/${mode}/`, search }}>Demo Assets</Link>
         <ThemeSwitcher />
       </header>
     </>
diff --git a/pages/app/components/index-page.tsx b/pages/app/components/index-page.tsx
index d8d6748af9..8afccbbf07 100644
--- a/pages/app/components/index-page.tsx
+++ b/pages/app/components/index-page.tsx
@@ -1,7 +1,7 @@
 // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 import React from 'react';
-import { Link } from 'react-router-dom';
+import { Link, useLocation } from 'react-router-dom';
 
 import { PACKAGE_VERSION } from '~components/internal/environment';
 import AwsUiLink from '~components/link';
@@ -38,10 +38,12 @@ function createPagesTree(pages: string[]) {
 }
 
 function TreeItemView({ item }: { item: TreeItem }) {
+  // Preserve the query string (theme, density, motion, …) when navigating to a page.
+  const { search } = useLocation();
   return (
     <li>
       {item.href ? (
-        <Link to={item.href} component={AwsUiLink}>
+        <Link to={{ pathname: item.href, search }} component={AwsUiLink}>
           {item.name}
         </Link>
       ) : (
diff --git a/pages/app/components/theme-switcher.tsx b/pages/app/components/theme-switcher.tsx
index 26ade0902b..7f7674d7f6 100644
--- a/pages/app/components/theme-switcher.tsx
+++ b/pages/app/components/theme-switcher.tsx
@@ -4,52 +4,47 @@ import React, { useContext } from 'react';
 
 import { Density, Mode } from '@cloudscape-design/global-styles';
 
-import { ALWAYS_VISUAL_REFRESH, INCLUDE_ONE_THEME } from '~components/internal/environment';
+import { ALWAYS_VISUAL_REFRESH } from '~components/internal/environment';
 import SpaceBetween from '~components/space-between';
 
 import AppContext from '../app-context';
+import { CLASSIC_THEME_ID, includedThemes, resolveActiveTheme } from '../theme-config';
 
 export default function ThemeSwitcher() {
   const { mode, urlParams, setUrlParams, setMode } = useContext(AppContext);
 
-  function activateTheme(theme: 'visualRefresh' | 'oneTheme' | 'classic') {
-    setUrlParams({
-      visualRefresh: theme === 'visualRefresh',
-      oneTheme: theme === 'oneTheme',
-    });
+  const activeTheme = resolveActiveTheme(urlParams.theme, Boolean(urlParams.visualRefresh));
+
+  function activateTheme(themeId: string) {
+    setUrlParams({ theme: themeId });
     window.location.reload();
   }
 
-  const vrSwitchProps: React.InputHTMLAttributes<HTMLInputElement> = {
-    id: 'visual-refresh-toggle',
-    type: 'checkbox',
-  };
-
-  if (ALWAYS_VISUAL_REFRESH) {
-    vrSwitchProps.checked = true;
-    vrSwitchProps.readOnly = true;
-  } else {
-    vrSwitchProps.checked = urlParams.visualRefresh && !urlParams.oneTheme;
-    vrSwitchProps.onChange = event => activateTheme(event.target.checked ? 'visualRefresh' : 'classic');
-  }
+  // Built from the themes compiled into this build. Classic is the baseline, except in
+  // always-visual-refresh builds where visual refresh is forced on and classic isn't selectable.
+  const themeOptions = [
+    ...(ALWAYS_VISUAL_REFRESH ? [] : [{ id: CLASSIC_THEME_ID, label: 'Classic' }]),
+    ...includedThemes,
+  ];
+  const selectedThemeId = activeTheme?.id ?? (ALWAYS_VISUAL_REFRESH ? 'visual-refresh' : CLASSIC_THEME_ID);
 
   return (
     <SpaceBetween direction="horizontal" size="xs">
       <label>
-        <input {...vrSwitchProps} />
-        Visual refresh
+        Theme{' '}
+        <select
+          id="theme-selector"
+          value={selectedThemeId}
+          disabled={ALWAYS_VISUAL_REFRESH}
+          onChange={event => activateTheme(event.target.value)}
+        >
+          {themeOptions.map(theme => (
+            <option key={theme.id} value={theme.id}>
+              {theme.label}
+            </option>
+          ))}
+        </select>
       </label>
-      {INCLUDE_ONE_THEME && (
-        <label>
-          <input
-            id="one-theme-toggle"
-            type="checkbox"
-            checked={urlParams.oneTheme}
-            onChange={event => activateTheme(event.target.checked ? 'oneTheme' : 'classic')}
-          />
-          One theme
-        </label>
-      )}
       <label>
         <input
           id="mode-toggle"
diff --git a/pages/app/index.tsx b/pages/app/index.tsx
index a1d89e692e..076f608252 100644
--- a/pages/app/index.tsx
+++ b/pages/app/index.tsx
@@ -14,6 +14,7 @@ import Header from './components/header';
 import IndexPage from './components/index-page';
 import PageView from './components/page-view';
 import StrictModeWrapper from './components/strict-mode-wrapper';
+import { includedThemes, resolveActiveTheme } from './theme-config';
 
 // import font-size reset and Ember font
 import '@cloudscape-design/global-styles/index.css';
@@ -95,12 +96,12 @@ function App() {
 }
 
 const history = createHashHistory();
-const { direction, visualRefresh, oneTheme, appLayoutWidget, appLayoutToolbar, appLayoutDelayedWidget } = parseQuery(
-  history.location.search
-);
+const query = parseQuery(history.location.search);
+const { direction, appLayoutWidget, appLayoutToolbar, appLayoutDelayedWidget } = query;
+
+const activeTheme = resolveActiveTheme(query.theme, Boolean(query.visualRefresh));
 
-// The VR class needs to be set before any React rendering occurs.
-window[awsuiVisualRefreshFlag] = () => visualRefresh && !oneTheme;
+window[awsuiVisualRefreshFlag] = () => activeTheme?.id === 'visual-refresh';
 if (!window[awsuiGlobalFlagsSymbol]) {
   window[awsuiGlobalFlagsSymbol] = {};
 }
@@ -111,10 +112,10 @@ window[awsuiGlobalFlagsSymbol].appLayoutWidget = appLayoutWidget;
 window[awsuiGlobalFlagsSymbol].appLayoutToolbar = appLayoutToolbar;
 window[awsuiCustomFlagsSymbol].appLayoutDelayedWidget = appLayoutDelayedWidget;
 
-// Visual Refresh and One Theme are mutually exclusive — manage both classes here so they never coexist.
-// useRuntimeVisualRefresh() detects .awsui-visual-refresh on body and short-circuits before its Symbol fallback.
-document.body.classList.toggle('awsui-one-theme', oneTheme);
-document.body.classList.toggle('awsui-visual-refresh', visualRefresh && !oneTheme);
+// Apply only the active theme's body class so themes don't coexist.
+for (const theme of includedThemes) {
+  document.body.classList.toggle(theme.bodyClass, theme.id === activeTheme?.id);
+}
 
 // Apply the direction value to the HTML element dir attribute
 document.documentElement.setAttribute('dir', direction);
diff --git a/pages/app/theme-config.ts b/pages/app/theme-config.ts
new file mode 100644
index 0000000000..aa702265a1
--- /dev/null
+++ b/pages/app/theme-config.ts
@@ -0,0 +1,36 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+import { INCLUDED_THEMES } from '~components/internal/environment';
+
+export interface SelectableTheme {
+  id: string;
+  label: string;
+  bodyClass: string;
+}
+
+const THEME_METADATA: Record<string, { label?: string }> = {
+  'visual-refresh': { label: 'Visual refresh' },
+  'one-theme': { label: 'One theme' },
+};
+
+// The classic baseline (no secondary theme) used as the `theme` URL param value.
+export const CLASSIC_THEME_ID = 'classic';
+
+export const includedThemes: SelectableTheme[] = INCLUDED_THEMES.map(id => ({
+  id,
+  label: THEME_METADATA[id]?.label ?? id,
+  bodyClass: `awsui-${id}`,
+}));
+
+function findIncludedTheme(id: string): SelectableTheme | null {
+  return includedThemes.find(theme => theme.id === id) ?? null;
+}
+
+// Resolves the active theme, preferring the canonical `theme` URL param. Falls back to the legacy
+// `visualRefresh` boolean param, kept for backwards compatibility.
+export function resolveActiveTheme(themeParam: string | undefined, isVisualRefresh: boolean): SelectableTheme | null {
+  if (themeParam) {
+    return themeParam === CLASSIC_THEME_ID ? null : findIncludedTheme(themeParam);
+  }
+  return isVisualRefresh ? findIncludedTheme('visual-refresh') : null;
+}
diff --git a/src/internal/environment.d.ts b/src/internal/environment.d.ts
index e7c3bc053f..81e24e1dea 100644
--- a/src/internal/environment.d.ts
+++ b/src/internal/environment.d.ts
@@ -9,3 +9,5 @@ export const PACKAGE_VERSION: string;
 export const GIT_SHA: string;
 /** Indicates that the current theme is always in visual refresh mode. */
 export const ALWAYS_VISUAL_REFRESH: boolean;
+/** Secondary themes compiled into this build (controlled by the THEMES env var). */
+export const INCLUDED_THEMES: string[];