How to check which line of code caused the http request?

I downloaded a wordpress theme from the Internet. When I used it, I found that the theme’s css file and js file were loaded from CDN acceleration. I wanted to change it to local direct loading. However, there is a css file that is always loaded from the cdn. Using keyword search, I cannot find the code that loads it.

<code><link rel='stylesheet' id='apsc-font-awesome-css'  href='https://maxcdn.bootstrapcdn.com/font-awesome/4.4.0/css/font-awesome.min.css?ver=1.6.5' type='text/css' media='all' /></code>

How to find out which piece of code is causing this?