AMP pages are not loading any CSS code from the Joomla template, given that the Google AMP package is distributed with a restricted set of default inline styles.
However, you can add a certain amount of custom CSS styles thanks to the JAmp parameter. This can be useful if you need to replicate similar styles to the default Joomla template, in this case you can simply copy/paste the required CSS code from the template into the JAmp textarea.
Let's say that you want to center the text of the post in an AMP page, you can add your CSS rule as shown below:
Keep in mind that there are certain restrictions when dealing with CSS code for an AMP page, for example you can't use the keyword '!important' and there is a 75,000 byte CSS limit.