ui: Adjust app bar and scaffold-background to align with new Figma design

From discussion:
  https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/design.3A.20colors/near/1801938

and the Figma linked there:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=2074%3A26325&t=rjuqcQaHMiGBUReF-1

Now, the app-bar bottom border is colored by the new Figma's
`border-bar`, and the main background color is `bg-main`. The app
bar background is unchanged because it already matches `bg-top-bar`.
I've labeled these values with comments in the code.

And all these color variables have dark-theme variants! (See
discussion for what those are.) That'll help when we implement dark
theme, #95.

Author: chrisbobbe

lib/widgets/theme.dart

@@ -12,9 +12,12 @@ ThemeData zulipThemeData(BuildContext context) {
       // ColorScheme.surfaceContainer for the scrolled-under state and
       // ColorScheme.surface otherwise, and those are different colors.
       scrolledUnderElevation: 0,
-      backgroundColor: Color(0xfff5f5f5),
+      backgroundColor: Color(0xfff5f5f5), // `bg-top-bar` in Figma
 
-      shape: Border(bottom: BorderSide(color: Color(0xffcccccc))),
+      shape: Border(bottom: BorderSide(
+        color: Color(0x33000000), // `border-bar` in Figma
+        strokeAlign: BorderSide.strokeAlignInside, // (default restated for explicitness)
+      )),
     ),
     // This applies Material 3's color system to produce a palette of
     // appropriately matching and contrasting colors for use in a UI.
@@ -27,7 +30,7 @@ ThemeData zulipThemeData(BuildContext context) {
     colorScheme: ColorScheme.fromSeed(
       seedColor: kZulipBrandColor,
     ),
-    scaffoldBackgroundColor: const Color(0xfff6f6f6),
+    scaffoldBackgroundColor: const Color(0xfff0f0f0), // `bg-main` in Figma
     tooltipTheme: const TooltipThemeData(preferBelow: false),
   );
 }