Use a constant for the default value of the theme

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/util/Preferences.java

@@ -393,7 +393,7 @@ public class Preferences {
                     // fall through
                 }
                 case 5: {
-                    setTheme(Constants.Pref.Theme.LIGHT);
+                    setTheme(Constants.Pref.Theme.DEFAULT);
                 }
             }