diff --git a/telemetry/ui/src/App.tsx b/telemetry/ui/src/App.tsx
index 121337ae3..d49900c49 100644
--- a/telemetry/ui/src/App.tsx
+++ b/telemetry/ui/src/App.tsx
@@ -31,6 +31,7 @@ import { StreamingChatbotWithTelemetry } from './examples/StreamingChatbot';
 import { AdminView } from './components/routes/AdminView';
 import { AnnotationsViewContainer } from './components/routes/app/AnnotationsView';
 import { DeepResearcherWithTelemetry } from './examples/DeepResearcher';
+import { useTheme } from './hooks/useTheme';
 
 /**
  * Basic application. We have an AppContainer -- this has a breadcrumb and a sidebar.
@@ -48,6 +49,9 @@ import { DeepResearcherWithTelemetry } from './examples/DeepResearcher';
  * @returns A rendered application object
  */
 const App = () => {
+  // Initialize theme at the app root so the `dark` class is applied on load
+  // (respects system preference, falls back to stored manual override).
+  useTheme();
   return (
     <QueryClientProvider client={new QueryClient()}>
       <Router basename={window.__BURR_BASE_PATH__ || ''}>
diff --git a/telemetry/ui/src/components/common/ThemeToggle.tsx b/telemetry/ui/src/components/common/ThemeToggle.tsx
new file mode 100644
index 000000000..569ce9061
--- /dev/null
+++ b/telemetry/ui/src/components/common/ThemeToggle.tsx
@@ -0,0 +1,47 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 { MoonIcon, SunIcon } from '@heroicons/react/24/outline';
+import { classNames } from '../../utils/tailwind';
+import { useTheme } from '../../hooks/useTheme';
+
+/**
+ * A simple sun/moon button that toggles between light and dark mode.
+ * Replaces the previously broken radio toggle referenced in issue #209.
+ */
+export const ThemeToggle = (props: { showLabel?: boolean }) => {
+  const { isDark, toggle } = useTheme();
+  const Icon = isDark ? SunIcon : MoonIcon;
+  const label = isDark ? 'Switch to light mode' : 'Switch to dark mode';
+  return (
+    <button
+      type="button"
+      onClick={toggle}
+      title={label}
+      aria-label={label}
+      className={classNames(
+        'group flex items-center gap-x-3 rounded-md p-2 text-sm leading-6 font-semibold',
+        'text-gray-700 hover:bg-gray-50 hover:text-dwdarkblue',
+        'dark:text-gray-200 dark:hover:bg-gray-800 dark:hover:text-white'
+      )}>
+      <Icon className="h-6 w-6 shrink-0 text-gray-400 group-hover:text-dwdarkblue dark:group-hover:text-white" />
+      {props.showLabel && <span>{isDark ? 'Light mode' : 'Dark mode'}</span>}
+    </button>
+  );
+};
diff --git a/telemetry/ui/src/components/nav/appcontainer.tsx b/telemetry/ui/src/components/nav/appcontainer.tsx
index 35c217e5f..5e4969b7d 100644
--- a/telemetry/ui/src/components/nav/appcontainer.tsx
+++ b/telemetry/ui/src/components/nav/appcontainer.tsx
@@ -37,6 +37,7 @@ import { classNames } from '../../utils/tailwind';
 import React from 'react';
 import { DefaultService } from '../../api';
 import { useQuery } from 'react-query';
+import { ThemeToggle } from '../common/ThemeToggle';
 
 // Define your GitHub logo SVG as a React component
 const GithubLogo = () => (
@@ -64,8 +65,8 @@ const ToggleOpenButton = (props: { open: boolean; toggleSidebar: () => void }) =
   return (
     <MinimizeMaximizeIcon
       className={classNames(
-        'text-gray-400',
-        'h-8 w-8 hover:bg-gray-50 rounded-md hover:cursor-pointer'
+        'text-gray-400 dark:text-gray-500',
+        'h-8 w-8 hover:bg-gray-50 dark:hover:bg-gray-800 rounded-md hover:cursor-pointer'
       )}
       aria-hidden="true"
       onClick={props.toggleSidebar}
@@ -181,7 +182,7 @@ export const AppContainer = (props: { children: React.ReactNode }) => {
 
   return (
     <>
-      <div className="h-screen w-screen overflow-x-auto">
+      <div className="h-screen w-screen overflow-x-auto bg-white dark:bg-gray-900">
         <Transition.Root show={smallSidebarOpen} as={Fragment}>
           <Dialog as="div" className="relative z-50 lg:hidden" onClose={setSmallSidebarOpen}>
             <Transition.Child
@@ -224,7 +225,7 @@ export const AppContainer = (props: { children: React.ReactNode }) => {
                     </div>
                   </Transition.Child>
                   {/* Sidebar component, swap this element with another sidebar if you like */}
-                  <div className="flex grow flex-col gap-y-5 overflow-y-auto bg-white px-6 pb-2 py-2">
+                  <div className="flex grow flex-col gap-y-5 overflow-y-auto bg-white px-6 pb-2 py-2 dark:bg-gray-900">
                     <div className="flex h-16 shrink-0 items-center">
                       <img
                         className="h-10 w-auto"
@@ -280,7 +281,7 @@ export const AppContainer = (props: { children: React.ReactNode }) => {
             sidebarOpen ? 'h-screen lg:fixed lg:inset-y-0 lg:z-50 lg:flex lg:w-72 lg:flex-col' : ''
           }`}>
           {/* Sidebar component, swap this element with another sidebar if you like */}
-          <div className="flex grow flex-col gap-y-5 overflow-y-auto border-r border-gray-200 bg-white px-6 py-2">
+          <div className="flex grow flex-col gap-y-5 overflow-y-auto border-r border-gray-200 bg-white px-6 py-2 dark:border-gray-700 dark:bg-gray-900">
             <div className="flex h-16 shrink-0 items-center">
               <img
                 className="h-12 w-auto"
@@ -299,14 +300,14 @@ export const AppContainer = (props: { children: React.ReactNode }) => {
                             to={item.href}
                             className={classNames(
                               isCurrent(item.href, item.linkType)
-                                ? 'bg-gray-50'
-                                : 'hover:bg-gray-50',
-                              'group flex gap-x-3 rounded-md p-2 text-sm leading-6 font-semibold text-gray-700'
+                                ? 'bg-gray-50 dark:bg-gray-800'
+                                : 'hover:bg-gray-50 dark:hover:bg-gray-800',
+                              'group flex gap-x-3 rounded-md p-2 text-sm leading-6 font-semibold text-gray-700 dark:text-gray-200'
                             )}
                             target={item.linkType === 'external' ? '_blank' : undefined}
                             rel={item.linkType === 'external' ? 'noreferrer' : undefined}>
                             <item.icon
-                              className="h-6 w-6 shrink-0 text-gray-400"
+                              className="h-6 w-6 shrink-0 text-gray-400 dark:text-gray-500"
                               aria-hidden="true"
                             />
                             {item.name}
@@ -318,12 +319,12 @@ export const AppContainer = (props: { children: React.ReactNode }) => {
                                 <Disclosure.Button
                                   className={classNames(
                                     isCurrent(item.href, item.linkType)
-                                      ? 'bg-gray-50'
-                                      : 'hover:bg-gray-50',
-                                    'flex items-center w-full text-left rounded-md p-2 gap-x-3 text-sm leading-6 font-semibold text-gray-700'
+                                      ? 'bg-gray-50 dark:bg-gray-800'
+                                      : 'hover:bg-gray-50 dark:hover:bg-gray-800',
+                                    'flex items-center w-full text-left rounded-md p-2 gap-x-3 text-sm leading-6 font-semibold text-gray-700 dark:text-gray-200'
                                   )}>
                                   <item.icon
-                                    className="h-6 w-6 shrink-0 text-gray-400"
+                                    className="h-6 w-6 shrink-0 text-gray-400 dark:text-gray-500"
                                     aria-hidden="true"
                                   />
                                   {item.name}
@@ -342,9 +343,9 @@ export const AppContainer = (props: { children: React.ReactNode }) => {
                                         to={subItem.href}
                                         className={classNames(
                                           isCurrent(subItem.href, subItem.linkType)
-                                            ? 'bg-gray-50'
-                                            : 'hover:bg-gray-50',
-                                          'block rounded-md py-2 pr-2 pl-9 text-sm leading-6 text-gray-700'
+                                            ? 'bg-gray-50 dark:bg-gray-800'
+                                            : 'hover:bg-gray-50 dark:hover:bg-gray-800',
+                                          'block rounded-md py-2 pr-2 pl-9 text-sm leading-6 text-gray-700 dark:text-gray-300'
                                         )}
                                         target={
                                           subItem.linkType === 'external' ? '_blank' : undefined
@@ -367,7 +368,8 @@ export const AppContainer = (props: { children: React.ReactNode }) => {
                 </li>
               </ul>
             </nav>
-            <div className="flex justify-start -mx-5">
+            <div className="flex justify-between items-center -mx-5 px-5">
+              <ThemeToggle />
               <ToggleOpenButton open={sidebarOpen} toggleSidebar={toggleSidebar} />
             </div>
           </div>
diff --git a/telemetry/ui/src/components/nav/breadcrumb.tsx b/telemetry/ui/src/components/nav/breadcrumb.tsx
index d2da9d14e..0ad63fad4 100644
--- a/telemetry/ui/src/components/nav/breadcrumb.tsx
+++ b/telemetry/ui/src/components/nav/breadcrumb.tsx
@@ -67,11 +67,13 @@ export const BreadCrumb = () => {
                     <path d="M5.555 17.776l8-16 .894.448-8 16-.894-.448z" />
                   </svg>
                   {isNullPK ? (
-                    <span className="ml-4 text-sm font-medium text-gray-200">no primary key</span>
+                    <span className="ml-4 text-sm font-medium text-gray-200 dark:text-gray-600">
+                      no primary key
+                    </span>
                   ) : (
                     <Link
                       to={page.href}
-                      className="ml-4 text-sm font-medium text-gray-500 hover:text-gray-700"
+                      className="ml-4 text-sm font-medium text-gray-500 hover:text-gray-700 dark:text-gray-400 dark:hover:text-gray-200"
                       aria-current={page.current ? 'page' : undefined}
                     >
                       {decodeURIComponent(page.name)}
diff --git a/telemetry/ui/src/components/routes/ProjectList.tsx b/telemetry/ui/src/components/routes/ProjectList.tsx
index 8ccabc836..916b133e2 100644
--- a/telemetry/ui/src/components/routes/ProjectList.tsx
+++ b/telemetry/ui/src/components/routes/ProjectList.tsx
@@ -66,10 +66,10 @@ export const ProjectListTable = (props: { projects: Project[]; includeAnnotation
           return (
             <TableRow
               key={project.id}
-              className="hover:bg-gray-50 cursor-pointer"
+              className="hover:bg-gray-50 dark:hover:bg-gray-800 cursor-pointer"
               onClick={() => navigate(`/project/${project.id}`)}
             >
-              <TableCell className="font-semibold text-gray-700">
+              <TableCell className="font-semibold text-gray-700 dark:text-gray-200">
                 <div className="flex flex-row gap-2">
                   {chipType !== undefined && <Chip label={chipType} chipType={chipType}></Chip>}
                   {projectName}
diff --git a/telemetry/ui/src/hooks/useTheme.ts b/telemetry/ui/src/hooks/useTheme.ts
new file mode 100644
index 000000000..7bf2676d7
--- /dev/null
+++ b/telemetry/ui/src/hooks/useTheme.ts
@@ -0,0 +1,101 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 { useCallback, useEffect, useState } from 'react';
+
+export type ThemePreference = 'light' | 'dark' | 'system';
+
+const STORAGE_KEY = 'burr-theme';
+
+const getSystemPrefersDark = (): boolean =>
+  typeof window !== 'undefined' &&
+  window.matchMedia &&
+  window.matchMedia('(prefers-color-scheme: dark)').matches;
+
+const getStoredPreference = (): ThemePreference => {
+  if (typeof window === 'undefined') {
+    return 'system';
+  }
+  const stored = window.localStorage.getItem(STORAGE_KEY);
+  if (stored === 'light' || stored === 'dark' || stored === 'system') {
+    return stored;
+  }
+  return 'system';
+};
+
+/**
+ * Applies (or removes) the `dark` class on the root <html> element based on the
+ * resolved theme. Tailwind is configured with darkMode: 'class', so this is what
+ * actually switches the styling.
+ */
+const applyDarkClass = (isDark: boolean) => {
+  const root = document.documentElement;
+  if (isDark) {
+    root.classList.add('dark');
+  } else {
+    root.classList.remove('dark');
+  }
+};
+
+/**
+ * Theme hook that respects system preference by default, allows a manual
+ * override, and persists the choice to localStorage.
+ *
+ * - `preference` is the user's stored choice ('light' | 'dark' | 'system').
+ * - `isDark` is the resolved value actually applied to the DOM.
+ * - `setPreference` updates and persists the choice.
+ * - `toggle` cycles between light and dark (collapsing 'system' to its resolved value first).
+ */
+export const useTheme = () => {
+  const [preference, setPreferenceState] = useState<ThemePreference>(getStoredPreference);
+  const [systemPrefersDark, setSystemPrefersDark] = useState<boolean>(getSystemPrefersDark);
+
+  // Keep track of the system preference so 'system' mode reacts to OS changes.
+  useEffect(() => {
+    if (!window.matchMedia) {
+      return;
+    }
+    const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
+    const handler = (event: MediaQueryListEvent) => setSystemPrefersDark(event.matches);
+    mediaQuery.addEventListener('change', handler);
+    return () => mediaQuery.removeEventListener('change', handler);
+  }, []);
+
+  const isDark = preference === 'system' ? systemPrefersDark : preference === 'dark';
+
+  // Apply the resolved theme to the DOM whenever it changes.
+  useEffect(() => {
+    applyDarkClass(isDark);
+  }, [isDark]);
+
+  const setPreference = useCallback((next: ThemePreference) => {
+    setPreferenceState(next);
+    if (next === 'system') {
+      window.localStorage.removeItem(STORAGE_KEY);
+    } else {
+      window.localStorage.setItem(STORAGE_KEY, next);
+    }
+  }, []);
+
+  const toggle = useCallback(() => {
+    setPreference(isDark ? 'light' : 'dark');
+  }, [isDark, setPreference]);
+
+  return { preference, isDark, setPreference, toggle };
+};
diff --git a/telemetry/ui/tailwind.config.js b/telemetry/ui/tailwind.config.js
index 015d883b0..75d0ec2c3 100644
--- a/telemetry/ui/tailwind.config.js
+++ b/telemetry/ui/tailwind.config.js
@@ -34,5 +34,5 @@ module.exports = {
   plugins: [
     require("tailwindcss-question-mark"),
   ],
-  darkMode: 'false',
+  darkMode: 'class',
 };