Follow these steps to add a custom CSS code in AMP Page Builder:
Step 1: Create a new page in the WordPress dashboard. Scroll down a little to see AMP Page builder click on Pre-built AMP Layout button.

Step 2: Once you click on the Pre-built AMP Layout button -> Click Advance option -> In this “Enter Style in Head” option you need to add custom CSS code and then click on the “Save” button like below the screenshot.

The output of custom CSS code in AMP Page Builder

That’s it for this tutorial!
We hope it helped you. If you have any feedback or questions, then please share them in the comments section below.
I changed something and in AMP page builder I can see only Content. My question is: Do you know where I can add button to change font size, color change etc.? I want to see layout directiory, save layout, import/export and advance. How I can doing it?
Hi Anna,
Will you please contact our technical team from here- https://ampforwp.com/support/ they will help you as soon as possible.