Theme Horse

Login to your admin dashboard and go to the Appearance > Customizer > Additional CSS and paste below custom css code:

@media only screen and (min-width: 768px) {
    .site-header, #secondary, #page:before {
        width: 20%;
    }
    #content {
        width: 80%;
    }
}

Note: Change the digit value above to modify the width of the sidebar and main body.