diff --git a/frontend/src/components/Chat/MessageList.styles.ts b/frontend/src/components/Chat/MessageList.styles.ts
index 7273fa158..f650686e2 100644
--- a/frontend/src/components/Chat/MessageList.styles.ts
+++ b/frontend/src/components/Chat/MessageList.styles.ts
@@ -20,13 +20,15 @@ export const useMessageListStyles = makeStyles({
     flexDirection: 'row-reverse',
   },
   messageContent: {
-    backgroundColor: tokens.colorNeutralBackground3,
+    backgroundColor: tokens.colorNeutralBackground1,
+    border: `1px solid ${tokens.colorNeutralStroke2}`,
     padding: tokens.spacingVerticalM,
     borderRadius: tokens.borderRadiusMedium,
     flex: 1,
   },
   userMessageContent: {
-    backgroundColor: tokens.colorBrandBackground,
+    backgroundColor: tokens.colorBrandBackground2,
+    border: `1px solid ${tokens.colorBrandStroke2}`,
   },
   messageText: {
     whiteSpace: 'pre-wrap',
diff --git a/frontend/src/components/Chat/MessageList.test.tsx b/frontend/src/components/Chat/MessageList.test.tsx
index fd68e15ab..58bf2d761 100644
--- a/frontend/src/components/Chat/MessageList.test.tsx
+++ b/frontend/src/components/Chat/MessageList.test.tsx
@@ -87,6 +87,50 @@ describe("MessageList", () => {
     expect(screen.getByText("Assistant message test")).toBeInTheDocument();
   });
 
+  describe("bubble class composition", () => {
+    // Regression guard for an earlier bug where the user-bubble background
+    // override silently lost to the assistant-bubble base style. The cause was
+    // string-concatenated class names — Griffel's atomic CSS doesn't dedupe
+    // conflicting properties unless mergeClasses is used. We assert here that
+    // the two bubble containers receive distinguishable className strings, so
+    // the override always has a chance to win.
+    it("renders user and assistant bubbles with distinct class signatures", () => {
+      render(
+        <TestWrapper>
+          <MessageList
+            messages={[
+              {
+                role: "user",
+                content: "u",
+                timestamp: new Date().toISOString(),
+              },
+              {
+                role: "assistant",
+                content: "a",
+                timestamp: new Date().toISOString(),
+              },
+            ]}
+          />
+        </TestWrapper>
+      );
+      const userBubble = screen.getByTestId("message-bubble-0");
+      const assistantBubble = screen.getByTestId("message-bubble-1");
+      // The two bubble class strings must differ — otherwise the user
+      // override silently lost (which was the original bug).
+      expect(userBubble.className).not.toBe(assistantBubble.className);
+      // Both bubbles must carry at least one style hook (catches a future
+      // refactor that drops the className entirely).
+      expect(userBubble.className.trim()).not.toBe('');
+      expect(assistantBubble.className.trim()).not.toBe('');
+      // The user bubble must carry at least one style hook the assistant
+      // bubble doesn't have — that's the override actually being applied.
+      const userClasses = userBubble.className.split(/\s+/).filter(Boolean);
+      const assistantClasses = new Set(assistantBubble.className.split(/\s+/).filter(Boolean));
+      const userOnly = userClasses.filter(c => !assistantClasses.has(c));
+      expect(userOnly.length).toBeGreaterThan(0);
+    });
+  });
+
   it("should handle messages with image attachments", () => {
     const messagesWithAttachments: Message[] = [
       {
diff --git a/frontend/src/components/Chat/MessageList.tsx b/frontend/src/components/Chat/MessageList.tsx
index 6e2a54086..77fdc740b 100644
--- a/frontend/src/components/Chat/MessageList.tsx
+++ b/frontend/src/components/Chat/MessageList.tsx
@@ -8,6 +8,7 @@ import {
   Button,
   Tooltip,
   Spinner,
+  mergeClasses,
 } from '@fluentui/react-components'
 import { ArrowDownloadRegular, ArrowReplyRegular, ArrowForwardRegular, ChatAddRegular, BranchForkRegular } from '@fluentui/react-icons'
 import { Message, MessageAttachment } from '../../types'
@@ -138,13 +139,16 @@ export default function MessageList({ messages, onCopyToInput, onCopyToNewConver
         return (
           <div
             key={index}
-            className={`${styles.message} ${isUser ? styles.userMessage : ''}`}
+            className={mergeClasses(styles.message, isUser && styles.userMessage)}
           >
             <Avatar
               name={avatarName}
               color={isUser ? 'colorful' : isSimulated ? 'steel' : 'brand'}
             />
-            <div className={`${styles.messageContent} ${isUser ? styles.userMessageContent : ''}`}>
+            <div
+              className={mergeClasses(styles.messageContent, isUser && styles.userMessageContent)}
+              data-testid={`message-bubble-${index}`}
+            >
               {/* Error rendering */}
               {message.error && (
                 <div className={styles.errorContainer}>