For #22851: Adjust theme based on the selected wallpaper.

app/src/main/java/org/mozilla/fenix/utils/Settings.kt

@@ -490,7 +490,7 @@ class Settings(private val appContext: Context) : PreferencesHolder {
         }
     }
 
-    val shouldUseDarkTheme by booleanPreference(
+    var shouldUseDarkTheme by booleanPreference(
         appContext.getPreferenceKey(R.string.pref_key_dark_theme),
         default = false
     )

app/src/main/java/org/mozilla/fenix/wallpapers/Wallpaper.kt

@@ -9,8 +9,8 @@ import org.mozilla.fenix.R
 /**
  * A enum that represents the available wallpapers and their states.
  */
-enum class Wallpaper(val drawable: Int) {
-    FIRST(R.drawable.wallpaper_1),
-    SECOND(R.drawable.wallpaper_2),
-    NONE(R.attr.homeBackground);
+enum class Wallpaper(val drawable: Int, val isDark: Boolean) {
+    FIRST(drawable = R.drawable.wallpaper_1, isDark = true),
+    SECOND(drawable = R.drawable.wallpaper_2, isDark = false),
+    NONE(drawable = R.attr.homeBackground, isDark = false);
 }

app/src/main/java/org/mozilla/fenix/wallpapers/WallpaperManager.kt

@@ -4,8 +4,11 @@
 
 package org.mozilla.fenix.wallpapers
 
+import android.content.Context
 import android.view.View
+import androidx.appcompat.app.AppCompatDelegate
 import mozilla.components.support.ktx.android.content.getColorFromAttr
+import org.mozilla.fenix.ext.asActivity
 import org.mozilla.fenix.utils.Settings
 
 /**
@@ -30,6 +33,38 @@ class WallpaperManager(private val settings: Settings) {
             wallpaperContainer.setBackgroundResource(newWallpaper.drawable)
         }
         currentWallpaper = newWallpaper
+
+        adjustTheme(wallpaperContainer.context)
+    }
+
+    private fun adjustTheme(context: Context) {
+        val mode = if (currentWallpaper != Wallpaper.NONE) {
+            if (currentWallpaper.isDark) {
+                updateThemePreference(useDarkTheme = true)
+                AppCompatDelegate.MODE_NIGHT_YES
+            } else {
+                updateThemePreference(useLightTheme = true)
+                AppCompatDelegate.MODE_NIGHT_NO
+            }
+        } else {
+            updateThemePreference(followDeviceTheme = true)
+            AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM
+        }
+
+        if (AppCompatDelegate.getDefaultNightMode() != mode) {
+            AppCompatDelegate.setDefaultNightMode(mode)
+            context.asActivity()?.recreate()
+        }
+    }
+
+    private fun updateThemePreference(
+        useDarkTheme: Boolean = false,
+        useLightTheme: Boolean = false,
+        followDeviceTheme: Boolean = false
+    ) {
+        settings.shouldUseDarkTheme = useDarkTheme
+        settings.shouldUseLightTheme = useLightTheme
+        settings.shouldFollowDeviceTheme = followDeviceTheme
     }
 
     /**