Try this url http://site.com/wp-login.php?action=lostpassword instead of /my-account/lost-password
You can set a class as default class, then it will be added with all the widgets by default .But in case the widgets are added before setting the default class, you need to click save button for each widgets.
Regards
can you please let me know the issue you are facing on using developers code. I have test it and seems works fine,
