compose: Compose-box border is subtle in dark mode

Updated the top border and added shadow effect to the top of
the compose box to ensure it is subtle in dark mode.

For reference, see the Figma design:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=5908-64038&t=alSmAmdRXFDwT4IT-1

Fixes: zulip#1207

Author: lakshya1goel

lib/widgets/compose_box.dart

@@ -22,6 +22,63 @@ import 'store.dart';
 import 'text.dart';
 import 'theme.dart';
 
+/// Compose-box styles that differ between light and dark theme.
+///
+/// These styles will animate on theme changes (with help from [lerp]).
+class ComposeBoxTheme extends ThemeExtension<ComposeBoxTheme> {
+  factory ComposeBoxTheme.light(BuildContext context) {
+    return ComposeBoxTheme._(
+      boxShadow: null,
+    );
+  }
+
+  factory ComposeBoxTheme.dark(BuildContext context) {
+    return ComposeBoxTheme._(
+      boxShadow: [BoxShadow(
+        color: DesignVariables.dark.bgTopBar,
+        offset: const Offset(0, -4),
+        blurRadius: 16,
+        spreadRadius: 0,
+      )],
+    );
+  }
+
+  ComposeBoxTheme._({
+    required this.boxShadow,
+  });
+
+  /// The [ComposeBoxTheme] from the context's active theme.
+  ///
+  /// The [ThemeData] must include [ComposeBoxTheme] in [ThemeData.extensions].
+  static ComposeBoxTheme of(BuildContext context) {
+    final theme = Theme.of(context);
+    final extension = theme.extension<ComposeBoxTheme>();
+    assert(extension != null);
+    return extension!;
+  }
+
+  final List<BoxShadow>? boxShadow;
+
+  @override
+  ComposeBoxTheme copyWith({
+    List<BoxShadow>? boxShadow,
+  }) {
+    return ComposeBoxTheme._(
+      boxShadow: boxShadow ?? this.boxShadow,
+    );
+  }
+
+  @override
+  ComposeBoxTheme lerp(ComposeBoxTheme other, double t) {
+    if (identical(this, other)) {
+      return this;
+    }
+    return ComposeBoxTheme._(
+      boxShadow: BoxShadow.lerpList(boxShadow, other.boxShadow, t)!,
+    );
+  }
+}
+
 const double _composeButtonSize = 44;
 
 /// A [TextEditingController] for use in the compose box.
@@ -1056,7 +1113,9 @@ class _ComposeBoxContainer extends StatelessWidget {
     //   the message list itself
     return Container(width: double.infinity,
       decoration: BoxDecoration(
-        border: Border(top: BorderSide(color: designVariables.borderBar))),
+        border: Border(top: BorderSide(color: designVariables.borderBar)),
+        boxShadow: ComposeBoxTheme.of(context).boxShadow,
+      ),
       // TODO(#720) try a Stack for the overlaid linear progress indicator
       child: Material(
         color: designVariables.composeBoxBg,

lib/widgets/theme.dart

@@ -175,7 +175,7 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
     bgMenuButtonActive: Colors.black.withValues(alpha: 0.2),
     bgMenuButtonSelected: Colors.black.withValues(alpha: 0.25),
     bgTopBar: const Color(0xff242424),
-    borderBar: Colors.black.withValues(alpha: 0.5),
+    borderBar: const Color(0xffffffff).withValues(alpha: 0.1),
     borderMenuButtonSelected: Colors.white.withValues(alpha: 0.1),
     btnLabelAttLowIntDanger: const Color(0xffff8b7c),
     btnLabelAttMediumIntDanger: const Color(0xffff8b7c),