What to use for a CodeIgniter Backend

Hello.

We are a web development firm and we have built our own custom backend ontop of Code Igniter. This is working well but I wanted to find out the following:

1. Are there standard backends (IE: Admin consoles) that people use?

2. I see there are many of these backend themes that can be bought but if my understanding is correct these need to be fully integrated into your code manually (IE: you have to tie in the HTML, CSS and JS, to all your backend logic, functionality and validation, etc...) Are there any off the shelf backends that work well and can be "plugged in"?

3. If the above is incorrect, is it correct to say that most people and companies custom build their own backend functionality / admin section?

What is the best practices and way to approach this?

I would love to hear what you guys use and if there are standard backends packed with functionality or if they are all custom coded.

Looking forward to the responses.