Thanks for your post.
We don't limit logins to specific IP address/domains right now. This is intentional so that students can access content from multiple devices/locations - meaning that they have the flexibility to take the course wherever and whenever they choose.
We do have options for selling access to courses in bulk, though. Check out this article for the best ways to handle that on Thinkific - http://help.thinkific.com/support/discussions/topics/313490
The HTML/CSS of your site pages are fully customizable using our Themes feature. And as part of the Advanced plan, you can also use our API to build your own features (here are our API docs for your reference).
If you have specific customizations that you'd like to make, please reach out to the support team using the HELP button on your admin dashboard. They'll be happy to discuss some options with you.