There really isn't much customization that you can do. The icon and title that appear there you already specify via the options/style properties. The only other customizable component is the background color and this is generally recommended to be the page background color after the app launches, so that's what we set it to. (An add-on could override that.) Beyond that, there isn't really anything you can change on the splash screen - it's controlled by the OS/browser installing the PWA.