Based off the digest email I’ve seen from your site, I’m assuming your site has a dark default color scheme. For that case, you can fix the issue that’s happening with the header background color with the iOS Gmail app by adding the following code to your site’s Admin / Customize / Email Style / CSS tab:
.digest-header {
background-image: linear-gradient(#111, #111);
}
Note that the colors added to the linear-gradient should both be the same. They should match the value that’s set for your default theme’s color palette’s “header background” color:
I think this is only a good approach for sites that have a dark color scheme set as the default. Details about why it works are here: Investigating activity summary email dark mode issues.