theme: Implement DesignVariables.dark

Author: chrisbobbe

lib/widgets/theme.dart

@@ -6,7 +6,7 @@ import 'stream_colors.dart';
 import 'text.dart';
 
 ThemeData zulipThemeData(BuildContext context) {
-  final designVariables = DesignVariables();
+  final designVariables = DesignVariables.light();
   return ThemeData(
     typography: zulipTypography(context),
     extensions: [ContentTheme.light(context), designVariables],
@@ -75,13 +75,27 @@ const kZulipBrandColor = Color.fromRGBO(0x64, 0x92, 0xfe, 1);
 /// For how to export these from the Figma, see:
 ///   https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=2945-49492&t=MEb4vtp7S26nntxm-0
 class DesignVariables extends ThemeExtension<DesignVariables> {
-  DesignVariables() :
-    bgTopBar = const Color(0xfff5f5f5),
-    borderBar = const Color(0x33000000),
-    icon = const Color(0xff666699),
-    mainBackground = const Color(0xfff0f0f0),
-    title = const Color(0xff1a1a1a),
-    streamColorSwatches = StreamColorSwatches.light;
+  factory DesignVariables.light() {
+    return DesignVariables._(
+      bgTopBar: const Color(0xfff5f5f5),
+      borderBar: const Color(0x33000000),
+      icon: const Color(0xff666699),
+      mainBackground: const Color(0xfff0f0f0),
+      title: const Color(0xff1a1a1a),
+      streamColorSwatches: StreamColorSwatches.light,
+    );
+  }
+
+  factory DesignVariables.dark() {
+    return DesignVariables._(
+      bgTopBar: const Color(0xff242424),
+      borderBar: Colors.black.withOpacity(0.41),
+      icon: const Color(0xff7070c2),
+      mainBackground: const Color(0xff1d1d1d),
+      title: const Color(0xffffffff),
+      streamColorSwatches: StreamColorSwatches.dark,
+    );
+  }
 
   DesignVariables._({
     required this.bgTopBar,