Hi. i've added it to this function like the following in Preload.js, it works, but people have to know that the change can been seen after a while on web browsers , it may be the cache or something. Thank you lunafromthemoon :) I've noticed that the change is applied to the stage but not on the languages menu screen (Demo: RenJSMultilanguage-master). That's ok for me, but some one may need the change here too.
create: function() {
//init game and start main menu
game.stage.backgroundColor = "#FFFFF";
game.state.add('init', init);
game.state.start('init');
}