When making a theme for a brand, requirements sometimes include custom back, and drawer button icons.

AppBar displays back, close, and drawer buttons automatically but these buttons show a default icon based on the platform that cannot be changed from any inherited themes. To change these icons, we have to provide the custom icon buttons in the AppBar on every page, and we also have to add additional conditions like to check if the scaffold has a drawer or if this route can be popped.

By letting AppBar inherit these icons from the theme, we can make development more convenient and reduce a lot of code.

If this is accepted, then I can make a PR for this issue.