I don’t know if this got addressed via another thread for you, but create a child theme to 2010, don’t edit it.
There are numerous reasons for this but you will guaranteed regret it if you can’t upgrade 2010, because the functions that are added in there over time are key to more than just the basics, they handle some other important stuff too. That’s gotta be considered part of the core of WordPress that you work around.
As for the spacing? I don’t think that’s a setting, but I think the key to solving your problem lies in the CSS of the style.css to edit the way those work.
I’m not sure how you center something like that in an automatic or dynamic way. I recently had a cutting edge theme that told me to keep changing the left indent until I was happy it was centered. Beats me. But I don’t think I know of any menus that are actually centered on a bar or spread out over the bar. Go figure. But if it is doable then it’s going to be some great CSS function.
And you might find a function that does what you want but might not officially space the items out organically….?
Not sure I helped there but seriously use a child theme, you just gotta! ??