There is no 1 Click ready-to-go implementation (like installing a theme in wordpress) although that is not a bad idea
Most admin templates are implemented in custom websites / cms by taking the necesary html blocks and connecting it with your own functionality so that when you give an admin panel to your clients it would have a nice user interface.
OpenCart, Wordpress etc have so many options especially in the admin section that it would need a custom Admin template created just for that to cover each specific option.
That doesn't mean that you can't start implementing an admin template into your own opencart, wordpress, etc. But it will take a lot of time even if you're an experienced developer.