It depends on the individual theme.
I think the style
#menunav holds the outer div for the menu so you can try adjusting that. If you are using a fixed width theme you can apply padding, margin or maybe change the text-align setting to force it further out.
If your using a liquid width theme, you'll need to apply padding or margin as a percentage e.g. 33%
Because extra menu items appear when your logged in as an admin you might want to/need to apply two different outer wrapper style depending on if if is_admin($admin) - but you should only really need to do that in the very unlikely event it pushed the admin menu items too far off to the right.