/* default dom id (particles-js) */
//particlesJS();

/* config dom id */
//particlesJS('dom-id');

/* config dom id + config particles params */
particlesJS('particles-js', {
	canvas: {
		color_hex_bg: '#15414e',
		opacity: 1
	},
	particles: {
	    color_hex: '#fff',
		opacity: 1,
		size: 2.5,
		size_random: true,
		nb: 200,
		anim: {
			speed: 2
		}
	},
	retina_detect: true
});
/* default dom id (particles-js) */
//particlesJS();

/* config dom id */
//particlesJS('dom-id');

/* config dom id (optional) + config particles params */
particlesJS('particles-js', {
	canvas: {
		color_hex_bg: '#15414e',
		opacity: 1
	},
	particles: {
	    color_hex: '#fff',
		opacity: 1,
		size: 2.5,
		size_random: true,
		nb: 200,
		anim: {
			speed: 2
		}
	},
	retina_detect: true
});

Update app.js