How to add CSS class to custom logo?

WordPress provide a filter hook to custom logo customization. The hook get_custom_logo is the filter. To change logo class, this code may help you.

add_filter( 'get_custom_logo', 'change_logo_class' );


function change_logo_class( $html ) {

    $html = str_replace( 'custom-logo', 'your-custom-class', $html );
    $html = str_replace( 'custom-logo-link', 'your-custom-class', $html );

    return $html;
}

Reference: How to change wordpress custom logo and logo link class