When you installed WP, you told it the URL of the site. Which is the old URL. It uses that info to make URLs for images, css, js, etc. So your page is looking for css, js , and images from a URL that is no longer good.
You have to tell WP that the URL is changed.
1. Add your old domain name as a parked domain. Go to WP using that URL and sign in as admin. Go to Settings->General and adjust your site's URL
OR
2. cPanel --> PHPMyAdmin
go to your WordPress database
"Browse" the "options" or "wp_options" table
Should be the second item, "siteurl" , edit to the correct value, don't forget the http://
Also, around #39 , "home", could be edited too