h1.site-title:hover {
    background: silver;
}

but you might want to use an image instead of a color. Create a small graphic in the right color and use it with the repeat-x and repeat-y properties to have it appear as you like.

When you get that right, the CSS would be

.site-title:hover {
    background-image: url('https://signedsilverlining.com/wp-content/uploads/2018/07/Silver-Line-through.jpg');
    background-repeat:repeat-x;
    background-position-y: center;
}

To add CSS: use the “Additional CSS” option in the customizer. https://codex.www.ads-software.com/CSS#Custom_CSS_in_WordPress

Learn to use the Chrome Developer Tools to help you see and test changes to your CSS.