How to make a responsive header?

theme finance ( http://pts.msk.ru )

Hello, try to use the following CSS code.

You can add CSS code in Customize → Additional CSS section.

@media (max-width: 667px) {
	#slideshow .text-slider .maintitle {
		font-size: 24px !important;
	}
}

Please feel free to ask any other questions that you might have.

Kind Regards, Roman.