Fix RNTester system bars background when edge-to-edge is enforced (facebook#51929)

Summary:
This PR fixes RNTester system bars background color to match the app one (not solid black).

## Changelog:

- [Internal] [Changed] - Fix RNTester app system bars color when edge-to-edge is enforced

Pull Request resolved: facebook#51929

Test Plan:
https://github.com/user-attachments/assets/8be0b721-6514-408f-81cd-2106ae7a17c4

Rollback Plan:

Reviewed By: javache

Differential Revision: D76352950

Pulled By: alanleedev

fbshipit-source-id: 474a81564570764a597aa995a0677617263338be

Author: zoontek

packages/rn-tester/android/app/src/main/java/com/facebook/react/uiapp/RNTesterActivity.kt

@@ -7,11 +7,12 @@
 
 package com.facebook.react.uiapp
 
+import android.content.res.Configuration
 import android.graphics.Color
-import android.graphics.drawable.ColorDrawable
 import android.os.Bundle
 import android.view.View
 import android.widget.FrameLayout
+import androidx.core.graphics.drawable.toDrawable
 import androidx.core.view.ViewCompat
 import androidx.core.view.WindowInsetsCompat
 import com.facebook.react.FBRNTesterEndToEndHelper
@@ -43,13 +44,28 @@ internal class RNTesterActivity : ReactActivity() {
         if (this::initialProps.isInitialized) initialProps else Bundle()
   }
 
+  // set background color so it will show below transparent system bars on forced edge-to-edge
+  private fun maybeUpdateBackgroundColor() {
+    val isDarkMode =
+        resources.configuration.uiMode and Configuration.UI_MODE_NIGHT_MASK ==
+            Configuration.UI_MODE_NIGHT_YES
+
+    val color =
+        if (isDarkMode) {
+          Color.rgb(11, 6, 0)
+        } else {
+          Color.rgb(243, 248, 255)
+        }
+
+    window?.setBackgroundDrawable(color.toDrawable())
+  }
+
   override fun onCreate(savedInstanceState: Bundle?) {
     super.onCreate(savedInstanceState)
 
     fullyDrawnReporter.addReporter()
+    maybeUpdateBackgroundColor()
 
-    // set background color so it will show below transparent system bars on forced edge-to-edge
-    this.window?.setBackgroundDrawable(ColorDrawable(Color.BLACK))
     // register insets listener to update margins on the ReactRootView to avoid overlap w/ system
     // bars
     getReactDelegate()?.getReactRootView()?.let { rootView ->
@@ -69,6 +85,13 @@ internal class RNTesterActivity : ReactActivity() {
     }
   }
 
+  override fun onConfigurationChanged(newConfig: Configuration) {
+    super.onConfigurationChanged(newConfig)
+
+    // update background color on UI mode change
+    maybeUpdateBackgroundColor()
+  }
+
   override fun createReactActivityDelegate() = RNTesterActivityDelegate(this, mainComponentName)
 
   override fun getMainComponentName() = "RNTesterApp"