I have a partner community with custom pages for login, home, and error. I want to also assign a custom VF page to the Under Construction scenario. How can I do it? In the Site detail page I see other error pages I can customise but not this one
I don't think you will be able to change it, even if you customize the default UnderConstruction page I believe it won't take the effect because declarative assigment do not change the result.
Please refer following link this might help you for better understanding.