To update the background from Figma's default page background color, you can modify the background colour of your Figma page to any solid color you would like to use as the background color for your emails.

This background color will be used for your email background color in any HTML exports from the Emailify plugin, so the Figma page background color that surrounds all of your Emailify frames on the page will be reflected as the same outer background color around your HTML email content when they're exported.