Do you have access to a local instance of Incorta to work with? Working locally so that you can see your changes in real time and make adjustments is the easiest way to customize. Once you have something that you are happy with, Support can work with you to get the css file and any image files deployed to the proper spot on the backend in Dev and later Prod. Barring that, you could modify the css sample file attached to the article and provide it to Support for deployment and do the testing in your Dev environment. You might need a few more iterations and it will definitely be slower to get to an end result, but that should work.
This all assumes that you are able to do the modifications to the css file yourself. You always have the option to work with Incorta PS if needed.
Let me know if this helps.