How to customize the width of the scrollbar of the content area?

You would need to manually edit the JS code for that. Please open "/js/kose.js" file and find this code:

$('#inner-content-container').niceScroll({
	cursorcolor : ThemeOptions.content_scrollbar_color,//'#fcda1c',
	cursorwidth : 3,
	cursorborder : 0,
	touchbehavior : false,
	autohidemode : true,
	hidecursordelay : 1000,
	scrollspeed : 100,
	//bouncescroll : true,
});

Just increase the value of "cursorwidth" here to make the scrollbar bigger. Then clear all the the cache and see the result.

Note that the theme uses "nicescroll JS" for the scrollbar. You can find more info about its API here: http://areaaperta.com/nicescroll/