You need to be in developer mode but you can style your maintenance pages by simply adding
maintanence.html in your root directory and style it like any custom html page
You have to be in production mode though it wont work in devleoper mode
See the docs here
You don't need to override it , but you can call your custom modal-popup.html file for your pop up.
Below is the code for creating pop up. I have called my custom modal-popup.html in js code.
<a href="#" id="click-me">Click Me</a>
<div id="popup-mpdal" >