Question about responsive website

Clearly the theme does not resize the header you are referring to. You will need to resize it responsively yourself.

Start learning about @media queries and screen sizes

Can you give me a little bit more information? Can you point me in the direction?

CSS Media Query.
https://www.w3schools.com/cssref/css3_pr_mediaquery.asp
https://www.w3schools.com/css/css_rwd_mediaqueries.asp

DO you know of a site that already has the code written so I can copy to place in the css sheet?

I am not very good with this either. But it should look something like this.

@media screen and (min-width: 480px) {
#site-banner.blogname .site-title a {
font-size:12px;
}
}
I don’t know what else you could do besides changing the font size.