diff --git base/win/dark_mode_support.cc base/win/dark_mode_support.cc
index 325bc70b6ba97..d4117dbb1d4d8 100644
--- base/win/dark_mode_support.cc
+++ base/win/dark_mode_support.cc
@@ -6,6 +6,7 @@
 
 #include <windows.h>
 
+#include "base/command_line.h"
 #include "base/native_library.h"
 #include "base/win/windows_version.h"
 
@@ -85,11 +86,20 @@ const DarkModeSupport& GetDarkModeSupport() {
   return dark_mode_support;
 }
 
+bool IsForcedLightMode() {
+  static bool kIsForcedLightMode =
+      base::CommandLine::ForCurrentProcess()->HasSwitch(
+          "force-light-mode");
+  return kIsForcedLightMode;
+}
+
 }  // namespace
 
 namespace base::win {
 
 bool IsDarkModeAvailable() {
+  if (IsForcedLightMode())
+    return false;
   auto& dark_mode_support = GetDarkModeSupport();
   return (dark_mode_support.allow_dark_mode_for_app ||
           dark_mode_support.set_preferred_app_mode) &&
diff --git ui/native_theme/native_theme_mac.mm ui/native_theme/native_theme_mac.mm
index 37a21af908449..bb3d9a1841156 100644
--- ui/native_theme/native_theme_mac.mm
+++ ui/native_theme/native_theme_mac.mm
@@ -47,6 +47,14 @@ bool IsDarkMode() {
 bool IsHighContrast() {
   return NSWorkspace.sharedWorkspace.accessibilityDisplayShouldIncreaseContrast;
 }
+
+bool IsForcedLightMode() {
+  static bool kIsForcedLightMode =
+      base::CommandLine::ForCurrentProcess()->HasSwitch(
+          "force-light-mode");
+  return kIsForcedLightMode;
+}
+
 }  // namespace
 
 // Helper object to respond to light mode/dark mode changeovers.
@@ -549,11 +557,15 @@ void NativeThemeMac::PaintSelectedMenuItem(
 
 void NativeThemeMac::InitializeDarkModeStateAndObserver() {
   __block auto theme = this;
-  set_use_dark_colors(IsDarkMode());
+  if (!IsForcedLightMode()) {
+    set_use_dark_colors(IsForcedDarkMode() || IsDarkMode());
+  }
   set_preferred_color_scheme(CalculatePreferredColorScheme());
   appearance_observer_.reset(
       [[NativeThemeEffectiveAppearanceObserver alloc] initWithHandler:^{
-        theme->set_use_dark_colors(IsDarkMode());
+        if (!IsForcedLightMode()) {
+          theme->set_use_dark_colors(IsForcedDarkMode() || IsDarkMode());
+        }
         theme->set_preferred_color_scheme(CalculatePreferredColorScheme());
         theme->NotifyOnNativeThemeUpdated();
       }]);
@@ -562,7 +574,9 @@ void NativeThemeMac::InitializeDarkModeStateAndObserver() {
 void NativeThemeMac::ConfigureWebInstance() {
   // NativeThemeAura is used as web instance so we need to initialize its state.
   NativeTheme* web_instance = NativeTheme::GetInstanceForWeb();
-  web_instance->set_use_dark_colors(IsDarkMode());
+  if (!IsForcedLightMode()) {
+    web_instance->set_use_dark_colors(IsForcedDarkMode() || IsDarkMode());
+  }
   web_instance->set_preferred_color_scheme(CalculatePreferredColorScheme());
   web_instance->SetPreferredContrast(CalculatePreferredContrast());
 
diff --git ui/native_theme/native_theme_win.cc ui/native_theme/native_theme_win.cc
index 87807be3558a2..adb672aee957b 100644
--- ui/native_theme/native_theme_win.cc
+++ ui/native_theme/native_theme_win.cc
@@ -621,14 +621,17 @@ bool NativeThemeWin::ShouldUseDarkColors() const {
   // Windows high contrast modes are entirely different themes,
   // so let them take priority over dark mode.
   // ...unless --force-dark-mode was specified in which case caveat emptor.
-  if (InForcedColorsMode() && !IsForcedDarkMode())
+  if (supports_windows_dark_mode_ && IsForcedDarkMode()) {
+    return true;
+  }
+  if (InForcedColorsMode())
     return false;
   return NativeTheme::ShouldUseDarkColors();
 }
 
 NativeTheme::PreferredColorScheme
 NativeThemeWin::CalculatePreferredColorScheme() const {
-  if (!InForcedColorsMode())
+  if (!InForcedColorsMode() || !supports_windows_dark_mode_)
     return NativeTheme::CalculatePreferredColorScheme();
 
   // According to the spec, the preferred color scheme for web content is 'dark'
@@ -1580,8 +1583,9 @@ void NativeThemeWin::RegisterThemeRegkeyObserver() {
 }
 
 void NativeThemeWin::UpdateDarkModeStatus() {
-  bool dark_mode_enabled = false;
-  if (hkcu_themes_regkey_.Valid()) {
+  bool dark_mode_enabled = ShouldUseDarkColors();
+  if (supports_windows_dark_mode_ && !IsForcedDarkMode() &&
+      hkcu_themes_regkey_.Valid()) {
     DWORD apps_use_light_theme = 1;
     hkcu_themes_regkey_.ReadValueDW(L"AppsUseLightTheme",
                                     &apps_use_light_theme);