This allows you to have a consistent CSS across thousands of landing pages ensuring your brand identity is consistent.
You can also add other analytics script so to track user behaviour on your landing pages so to improve conversion metrics.
Lets follow the below steps to do this:
Go to the specific landing page edit mode from Landing Pages > Specific Landing Page > Edit
Go to Design Section > Custom CSS and Custom JS
Once you are satisfied with the changes, you can click submit and you are done.