Android: Add black backgrounds toggle

Makes all background colors black in dark mode when enabled through a ThemeOverlay. Applied the same way as a theme/mode.

Source/Android/app/src/main/res/values-night/themes.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <style name="ThemeOverlay.Dolphin.Dark" parent="">
+        <item name="colorSurface">@color/background_black</item>
+        <item name="android:colorBackground">@color/background_black</item>
+    </style>
+</resources>

Source/Android/app/src/main/res/values/dolphin_colors.xml

@@ -39,4 +39,6 @@
     <color name="tv_card_unselected">#444444</color>
 
     <color name="invalid_setting_overlay">#36ff0000</color>
+
+    <color name="background_black">#000000</color>
 </resources>

Source/Android/app/src/main/res/values/strings.xml

@@ -197,6 +197,8 @@
     <string name="show_titles_in_game_list_description">Show the title and creator below each game cover.</string>
     <string name="change_theme">Change App Theme</string>
     <string name="change_theme_mode">Change Theme Mode</string>
+    <string name="use_black_backgrounds">Use Black Backgrounds</string>
+    <string name="use_black_backgrounds_description">When using the dark theme, apply black backgrounds.</string>
 
     <!-- Online Update Region Select Fragment -->
     <string name="region_select_title">Please select a region</string>