how to change font size in header

Hi,

To change the font size in the header go Customize > Appearance > Additional CSS and paste the following (the size is just an example, put the one you want. Note that 14px = 1rem):

To change the site title:

.blog-info-sin-imagen h1.site-title{
	font-size:42px !important;
	font-size:3rem !important;
}

To change the site description:

h2.site-description{
	font-size: 18px;
	font-size: 1.285714285714286rem;
}

Regards.