Although you can manually customize each aspect of the notification bar, you can also choose any of the preset themes. You can also change your avatar image directly from the Material Notification Shade options.
Material Notification Shade is an excellent customization app, and doesn't require any root permissions. If you have a rooted smartphone, you can also access some additional features, but it's not a requirement in order to use the app.
Requires Android 5.0 or higher