How to Integrate Bootstrap v4 NavBar into WordPress theme?

I am using Bootstrap 4 and my menu structure like this:

Let’s start adding menu dynamically on wordpress custom theme. I am calling WP Nav in header.php file like this: Now, add this function in function.php file.

How to vertically center a div for all browsers?

Below is the best all-around solution I have found. Tested and working for all browser.