Fix reader background color (fixes #2694)

app/src/main/res/values/themes.xml

@@ -151,6 +151,7 @@
         <item name="colorPrimaryDark">@color/readerColorDarkPrimary</item>
         <item name="colorSurface">@android:color/black</item>
 
+        <item name="android:colorBackground">@android:color/black</item>
         <item name="android:statusBarColor">?attr/colorPrimaryDark</item>
         <item name="android:navigationBarColor">?attr/colorPrimaryDark</item>
     </style>
@@ -160,6 +161,7 @@
         <item name="colorPrimaryDark">@color/readerColorDarkPrimary</item>
         <item name="colorSurface">@android:color/white</item>
 
+        <item name="android:colorBackground">@android:color/white</item>
         <item name="android:statusBarColor">?attr/colorPrimaryDark</item>
         <item name="android:navigationBarColor">?attr/colorPrimaryDark</item>
     </style>