diff --git a/lib/widgets/theme.dart b/lib/widgets/theme.dart
index ac6ff28f13..c8742cae72 100644
--- a/lib/widgets/theme.dart
+++ b/lib/widgets/theme.dart
@@ -5,11 +5,50 @@ import 'content.dart';
 import 'stream_colors.dart';
 import 'text.dart';
 
+/// In debug mode, controls whether the UI responds to
+/// [MediaQueryData.platformBrightness].
+///
+/// Outside of debug mode, this is always false and the setter has no effect.
+// TODO(#95) when dark theme is fully implemented, simplify away;
+//   the UI should always respond.
+bool get debugFollowPlatformBrightness {
+  bool result = false;
+  assert(() {
+    result = _debugFollowPlatformBrightness;
+    return true;
+  }());
+  return result;
+}
+bool _debugFollowPlatformBrightness = false;
+set debugFollowPlatformBrightness(bool value) {
+  assert(() {
+    _debugFollowPlatformBrightness = value;
+    return true;
+  }());
+}
+
+
 ThemeData zulipThemeData(BuildContext context) {
-  final designVariables = DesignVariables();
+  final DesignVariables designVariables;
+  final List<ThemeExtension> themeExtensions;
+  Brightness brightness = debugFollowPlatformBrightness
+    ? MediaQuery.of(context).platformBrightness
+    : Brightness.light;
+  switch (brightness) {
+    case Brightness.light: {
+      designVariables = DesignVariables.light();
+      themeExtensions = [ContentTheme.light(context), designVariables];
+    }
+    case Brightness.dark: {
+      designVariables = DesignVariables.dark();
+      themeExtensions = [ContentTheme.dark(context), designVariables];
+    }
+  }
+
   return ThemeData(
+    brightness: brightness,
     typography: zulipTypography(context),
-    extensions: [ContentTheme.light(context), designVariables],
+    extensions: themeExtensions,
     appBarTheme: AppBarTheme(
       // Set these two fields to prevent a color change in [AppBar]s when
       // there is something scrolled under it. If an app bar hasn't been
@@ -57,9 +96,10 @@ ThemeData zulipThemeData(BuildContext context) {
     // Or try this tool to see the whole palette:
     //   https://m3.material.io/theme-builder#/custom
     colorScheme: ColorScheme.fromSeed(
+      brightness: brightness,
       seedColor: kZulipBrandColor,
     ),
-    scaffoldBackgroundColor: designVariables.bgMain,
+    scaffoldBackgroundColor: designVariables.mainBackground,
     tooltipTheme: const TooltipThemeData(preferBelow: false),
   );
 }
@@ -75,19 +115,31 @@ 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() :
-    bgMain = const Color(0xfff0f0f0),
-    bgTopBar = const Color(0xfff5f5f5),
-    borderBar = const Color(0x33000000),
-    icon = const Color(0xff666699),
-    title = const Color(0xff1a1a1a),
-    streamColorSwatches = StreamColorSwatches.light;
+  DesignVariables.light() :
+    this._(
+      bgTopBar: const Color(0xfff5f5f5),
+      borderBar: const Color(0x33000000),
+      icon: const Color(0xff666699),
+      mainBackground: const Color(0xfff0f0f0),
+      title: const Color(0xff1a1a1a),
+      streamColorSwatches: StreamColorSwatches.light,
+    );
+
+  DesignVariables.dark() :
+    this._(
+      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.bgMain,
     required this.bgTopBar,
     required this.borderBar,
     required this.icon,
+    required this.mainBackground,
     required this.title,
     required this.streamColorSwatches,
   });
@@ -102,10 +154,10 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
     return extension!;
   }
 
-  final Color bgMain;
   final Color bgTopBar;
   final Color borderBar;
   final Color icon;
+  final Color mainBackground;
   final Color title;
 
   // Not exactly from the Figma design, but from Vlad anyway.
@@ -113,18 +165,18 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
 
   @override
   DesignVariables copyWith({
-    Color? bgMain,
     Color? bgTopBar,
     Color? borderBar,
     Color? icon,
+    Color? mainBackground,
     Color? title,
     StreamColorSwatches? streamColorSwatches,
   }) {
     return DesignVariables._(
-      bgMain: bgMain ?? this.bgMain,
       bgTopBar: bgTopBar ?? this.bgTopBar,
       borderBar: borderBar ?? this.borderBar,
       icon: icon ?? this.icon,
+      mainBackground: mainBackground ?? this.mainBackground,
       title: title ?? this.title,
       streamColorSwatches: streamColorSwatches ?? this.streamColorSwatches,
     );
@@ -136,10 +188,10 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       return this;
     }
     return DesignVariables._(
-      bgMain: Color.lerp(bgMain, other.bgMain, t)!,
       bgTopBar: Color.lerp(bgTopBar, other.bgTopBar, t)!,
       borderBar: Color.lerp(borderBar, other.borderBar, t)!,
       icon: Color.lerp(icon, other.icon, t)!,
+      mainBackground: Color.lerp(mainBackground, other.mainBackground, t)!,
       title: Color.lerp(title, other.title, t)!,
       streamColorSwatches: StreamColorSwatches.lerp(streamColorSwatches, other.streamColorSwatches, t),
     );
diff --git a/test/widgets/theme_test.dart b/test/widgets/theme_test.dart
index 763aab5d11..b6d3c98a68 100644
--- a/test/widgets/theme_test.dart
+++ b/test/widgets/theme_test.dart
@@ -76,30 +76,74 @@ void main() {
       button: TextButton(onPressed: () {}, child: const Text(buttonText)));
   });
 
-  group('colorSwatchFor', () {
-    void doTest(String description, int baseColor, StreamColorSwatch expected) {
-      testWidgets('$description $baseColor', (WidgetTester tester) async {
-        addTearDown(testBinding.reset);
-
-        final subscription = eg.subscription(eg.stream(), color: baseColor);
+  group('DesignVariables', () {
+    group('lerp', () {
+      testWidgets('light -> light', (tester) async {
+        final a = DesignVariables.light();
+        final b = DesignVariables.light();
+        check(() => a.lerp(b, 0.5)).returnsNormally();
+      });
 
-        await tester.pumpWidget(const ZulipApp());
-        await tester.pump();
+      testWidgets('light -> dark', (tester) async {
+        final a = DesignVariables.light();
+        final b = DesignVariables.dark();
+        check(() => a.lerp(b, 0.5)).returnsNormally();
+      });
 
-        late StreamColorSwatch actualSwatch;
-        final navigator = await ZulipApp.navigator;
-        navigator.push(MaterialWidgetRoute(page: Builder(builder: (context) {
-          actualSwatch = colorSwatchFor(context, subscription);
-          return const Placeholder();
-        })));
-        await tester.pumpAndSettle();
+      testWidgets('dark -> light', (tester) async {
+        final a = DesignVariables.dark();
+        final b = DesignVariables.light();
+        check(() => a.lerp(b, 0.5)).returnsNormally();
+      });
 
-        // Compares all the swatch's members; see [ColorSwatch]'s `operator ==`.
-        check(actualSwatch).equals(expected);
+      testWidgets('dark -> dark', (tester) async {
+        final a = DesignVariables.dark();
+        final b = DesignVariables.dark();
+        check(() => a.lerp(b, 0.5)).returnsNormally();
       });
-    }
+    });
+  });
 
-    doTest('light', 0xff76ce90, StreamColorSwatch.light(0xff76ce90));
-    // TODO(#95) test with Brightness.dark and lerping between light/dark
+  group('colorSwatchFor', () {
+    const baseColor = 0xff76ce90;
+
+    testWidgets('light–dark animation', (WidgetTester tester) async {
+      addTearDown(testBinding.reset);
+
+      final subscription = eg.subscription(eg.stream(), color: baseColor);
+
+      assert(!debugFollowPlatformBrightness); // to be removed with #95
+      debugFollowPlatformBrightness = true;
+      addTearDown(() { debugFollowPlatformBrightness = false; });
+      tester.platformDispatcher.platformBrightnessTestValue = Brightness.light;
+      addTearDown(tester.platformDispatcher.clearPlatformBrightnessTestValue);
+
+      await tester.pumpWidget(const ZulipApp());
+      await tester.pump();
+
+      final navigator = await ZulipApp.navigator;
+      navigator.push(MaterialWidgetRoute(page: Builder(builder: (context) =>
+        const Placeholder())));
+      await tester.pumpAndSettle();
+
+      final element = tester.element(find.byType(Placeholder));
+      // Compares all the swatch's members; see [ColorSwatch]'s `operator ==`.
+      check(colorSwatchFor(element, subscription))
+        .equals(StreamColorSwatch.light(baseColor));
+
+      tester.platformDispatcher.platformBrightnessTestValue = Brightness.dark;
+      await tester.pump();
+
+      await tester.pump(kThemeAnimationDuration * 0.4);
+      check(colorSwatchFor(element, subscription))
+        .equals(StreamColorSwatch.lerp(
+          StreamColorSwatch.light(baseColor),
+          StreamColorSwatch.dark(baseColor),
+          0.4)!);
+
+      await tester.pump(kThemeAnimationDuration * 0.6);
+      check(colorSwatchFor(element, subscription))
+        .equals(StreamColorSwatch.dark(baseColor));
+    });
   });
 }