Change background color?

Can you please share the URL of your website? So that we will provide you the code by checking it.

Can you please add the below CSS in the WordPress Dashboard -> Navigate to AMP Option Panel -> Design -> Global -> In the Advanced section Paste your Custom CSS and then click on the “Save Changes” button and let us know back?

body {
    background: black;
}

p, h1, h2, h3, h4, h5, h6 span, a {
    color: white;
}

Can you please refer to the following tutorial and let us know back if you need any assistance?

Reference tutorial: https://ampforwp.com/tutorials/article/how-to-add-color-and-fonts-in-header-and-footer/

Glad to hear it is solved. If you have any other queries or issues, Please create a new topic.