Since last update (1.2.4), my menu, which was centered before (set in graphics settings), is always left-aligned, no matter what value I choose in the graphics settings.
No CSS is overloading this.
You copied the entire Nirvana style.css file to your child theme – this is not to correct method. The styling is also from an old 1.0.5 version – plenty of things changed since then.
Only include the styling rules that you are customizing in your child theme.