This is not nginx or even apache problem, is the code on your website, check all links and css, js files are being called using relative paths ie: /css/style.css (on your site code/php/html, not on the output you receive in your browser as this one will always be show as full path).

Also if you are calling external resources like js use //host.com/resource.js instead of http://host.com/resource.js so your site call it https or http when requested by the browser.