Use a constant for the default value of the theme

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/Constants.java

@@ -108,6 +108,7 @@ public final class Constants {
         public static final class Theme {
             public static final String LIGHT = "light";
             public static final String DARK = "dark";
+            public static final String DEFAULT = Constants.Pref.Theme.LIGHT;
         }
     }
 

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);
                 }
             }