Move system_overlay_style from AppBar to Theme

[ndonkoHenri]

Test code:

import flet as ft

def main(page: ft.Page):
    page.theme = ft.Theme(
        system_overlay_style=ft.SystemOverlayStyle(
            status_bar_color=ft.colors.ORANGE_100,
            system_navigation_bar_color=ft.colors.GREEN_300,
            system_navigation_bar_divider_color=ft.colors.BLUE,
            status_bar_icon_brightness=ft.Brightness.DARK,
            system_navigation_bar_icon_brightness=ft.Brightness.DARK,
            status_bar_brightness=ft.Brightness.LIGHT,
        ),
    )
    page.dark_theme = ft.Theme(
        system_overlay_style=ft.SystemOverlayStyle(
            status_bar_color=ft.colors.ORANGE,
            system_navigation_bar_color=ft.colors.GREEN_800,
            system_navigation_bar_divider_color=ft.colors.BLUE,
            status_bar_icon_brightness=ft.Brightness.LIGHT,
            system_navigation_bar_icon_brightness=ft.Brightness.LIGHT,
            status_bar_brightness=ft.Brightness.DARK,
        ),
    )
    page.theme_mode = ft.ThemeMode.LIGHT

    def dark_light_mode_changed(e):
        if page.theme_mode == ft.ThemeMode.LIGHT:
            page.theme_mode = ft.ThemeMode.DARK
        else:
            page.theme_mode = ft.ThemeMode.LIGHT
        page.update()

    dark_mode_checkbox = ft.Switch(
        value=False, label="Dark Mode", on_change=dark_light_mode_changed
    )

    page.appbar = ft.AppBar(
        leading=ft.Icon(ft.icons.STYLE),
        title=ft.Text("Using SystemOverlayStyle"),
        bgcolor=ft.colors.BLUE_500,
    )
    page.add(ft.SafeArea(ft.Column([ft.Text("Body!"), dark_mode_checkbox])))


ft.app(target=main)