Can I get widget change background color of simple way in modern gtk3?

["michinari.nukazawa" <michinari nukazawa gmail com>]

Hello.

Old day.
I should have done writing code of simple one line if I demanded the 
change of the background color
But it does not seem to be so now.
and nothing (simple) example code.
I see gtk3-demo. Using callback and other. not one line.

gtk_widget_modify_bg(window, GTK_STATE_NORMAL, &color);
-> is deprecated

gtk_widget_override_background_color(window, GTK_STATE_NORMAL, &color);
-> is deprecated

Probably modern way is CSS Style.
But nothing example.
and ways seem to be different until now.


Can I get modern gtk3 example?
Can I change background color by writing code of simple one line in 
modern gtk3?

Thanks.