Update PWA branding

With PWA you get app like styles/branding when saving the app to your home screen, these parameters are set in manifest.webmanifest. Update the styles there to make it look good. I'm sure exactly what this looks like, but I'm guessing theme_color and background_color should be our theme blue. This is based on what it currently looks like when you save to your home screen and open the app. These styles should give us a themed loading screen.