Set the theme to the default one if the user-defined one is invalid

Related to #1283

src/app/ui/skin/skin_theme.cpp

@@ -203,7 +203,8 @@ void SkinTheme::onRegenerate()
         Console::showException(e);
 
       // We can continue, as we've already loaded the default theme
-      // anyway...
+      // anyway. Here we restore the setting to its default value.
+      pref.theme.selected(pref.theme.selected.defaultValue());
     }
   }
 }