Center Menu

Centring nav menus has been asked and answered many times before;

• http://wordpress.org/support/topic/centering-nav-menu-in-slim-theme?replies=3
• http://wordpress.org/support/topic/gridiculous-menu-site-navigation-cant-center?replies=5
• http://wordpress.org/support/topic/twenty-twelve-theme-center-align-items-main-menu?replies=5
• http://wordpress.org/support/topic/delicate-theme-how-to-center-navigation-bar?replies=7
• http://wordpress.org/support/topic/how-to-put-header-and-menubar-in-center?replies=3
• http://wordpress.org/support/topic/show-menu-in-center?replies=3
• http://wordpress.org/support/topic/center-menu-help?replies=14

Try using a browser developer tool for this kind of exploratory CSS work. You can expose↓ styles that are applying to elements in your webpage.
I use Google Chrome’s built-in Developer Tool because I find it particularly easy to use.

Alternatively, ask general CSS forums;
http://csscreator.com/forum
http://www.cssforum.com.pk/
http://css-tricks.com/forums/
http://www.welovecss.com/
http://www.codingforums.com/

Now that you know the style, from being exposed by the Developer Tool/ recommended by the CSS forum, you can add↓ a new style to override it.

You ought to add new styles externally↓, not within the theme’s files. Changes to the theme’s files will compromise the theme and be compromised when the theme updates.

External changes can be made using a Child Theme or through a plugin that provides Custom CSS↓ functionality.

I recommend using the Jetpack plugin’s Custom CSS functionality to hold your CSS.