Centering NavBar

I have read the many threads of those who have gotten help with centering the navbar, and I have tried the tricks, but I am not having very much luck. I emailed the creator of the theme, but since I am not purchasing it, he could only give so much help but he did tell me I would have to play with the .navbar ul li {} part of the css code.

Now I have tried the text-align;center and everything else, and nothing seems to be working.

My website is https://www.memphiscofchrist.org. And any help would be very much appreciated.

Thank you.