How to make a navigation bar fix after a center amount of scrolling?

This article explains how to do it. You can ignore the first part about HTML, since that’s already create by your theme, although you will need to know the ID or class of your navigation bar. It’s easy enough that I implemented it on one of my sites. Basically, you need to add some JavaScript (actually jQuery) that will 1) detect when the screen has been scrolled a certain number of pixels, and 2) add a class to the navigation bar once that threshold has been reached. Then, you add a CSS rule which sets the menu bar’s position to fixed for that particular class.

If your theme does not have an option to add script code or CSS, then you can install Header and Footer to add your script code and Custom CSS Manager to add the CSS.