From Sam:
Integrating the browser-picker / FRE / offline error pages to some extent with the brand & feel of an app would help make it feel like you were really using an app and not looking at a Chrome error page.

I know the manifest has got an image and a theme color and I believe we use that to construct the splash screen, but is it possible to get that info from the WebAPK itself? In the offline case especially we have to assume that Chrome has absolutely nothing in its cache from this site. The WebAPK has the icon, but is it feasible to use it in an error screen? Do we know the theme color at all?