How to Change Font Size on One Page?

Link to the page?

https://www.qualityfoaminc.com/mattresses/

You have this being generated:

ARTICLE DIV DIV.gallery.galleryid-51.gallery-columns-3.gallery-size-medium DL.gallery-item DD#gallery-1-64.wp-caption-text.gallery-caption, ARTICLE DIV DIV.gallery.galleryid-51.gallery-columns-3.gallery-size-medium DL.gallery-item DD#gallery-1-71.wp-caption-text.gallery-caption, ARTICLE DIV DIV.gallery.galleryid-51.gallery-columns-3.gallery-size-medium DL.gallery-item DD#gallery-1-70.wp-caption-text.gallery-caption {
font-size: 16px !important;
font-family:'gothambold1' !important;}
DIV HEADER DIV.brand.span3.pull-left H1 A.site-title {
font-family:'toledoheavysfbold0' !important;}

which needs to be removed.

You’d then be able to customise:

.entry-caption, .tc-gallery-style .gallery-caption, .wp-caption .wp-caption-text {
    font-style: italic; /* Change to normal */
    font-size: 12px;
    font-size: .7rem; /* Change to preferred */
    line-height: 1.3em;
    color: #757575; /* Change to preferred */
    font-family: 'Comic Sans' /* Change to preferred */
}

I wish I could apply this code to any page and any element. Hoe do I change the font on only the main title without using the Google fonts fontpicker?
https://www.qualityfoaminc.com/

The page is using this:

.site-title,.site-description,h1,h2,h3,.tc-dropcap {
                  font-family : 'Dosis';
                  font-weight : 400;
                }

You can change using:

.site-title {
    font-family: '**YOURFONT**';
    font-weight: 400;
}

Where do I add that code?

Preferably a Child Theme style.css, but test in Customize>Advanced options>Custom CSS