Moving the top bar menu to the center

Some one gave me the solution. But I have no idea were to put it.
I am very new to css and html. So that is why I am having so much trouble for what looks like an easy fix.

https://mrslavic.com.transurl.nl/

“this seems to work (in refox with web develper add-on):

#main-menu { display: block; text-align: center; }
.sm-clean li { float: none; display: inline-block; }”