Custom new tab on new tab extension

I'm currently using the new tab extension, the experience is good, but I would like to improve it more.

Is there a way I can use by default bookmark search, fallback to normal search if no bookmark found?
Can I create a custom new tab page by my own and open vomnibar on start?
Is there anyway to see how the blank new tab page code is, how the service workers cache works?
can I customize the omnibar css styles?