Hi there. This is a complicated CSS change and beyond the realm of basic support. However, this is how I’d approach it:
You’d need to set a fixed width on the menu based on the number of items in the menu (note that this would have to be changed every time you wanted to add or remove a menu item, so it’s not fluid), set margins to auto and display type to block. I’m not sure how this affects the drop-down functionality; you’d have to play around with it. Look at a guide like this to get an idea, or Google “center horizontal menu” for other tutorials.
As always, I recommend making these changes within a child theme so your changes aren’t overwritten by future theme updates.