add black background option

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

@@ -52,6 +52,7 @@
     <string name="preference_theme_default">System default</string>
     <string name="preference_theme_light">Light</string>
     <string name="preference_theme_dark">Dark</string>
+    <string name="preference_black_background_name">Black background in dark theme</string>
 
     <string name="preference_category_updater">Updates</string>
     <string name="preference_update_automatically_name">Enable automatic updates</string>