Get rid of the widget Item Class=”widget-item”

The most straightforward way would be to overwrite this in your CSS file, for instance like this:

.widgettitle {
    padding: 0;
    margin: 0;
}

Alternatively you can change the name of the class when registering your widget (possibly in your functions.php file) – look for the 'before_title' parameter. Here’s and example form the original twentysexteen template:

register_sidebar( array(
        'name'          => __( 'Sidebar', 'twentysixteen' ),
        'id'            => 'sidebar-1',
        'description'   => __( 'Add widgets here to appear in your sidebar.', 'twentysixteen' ),
        'before_widget' => '<section id="%1$s" class="widget %2$s">',
        'after_widget'  => '</section>',
        'before_title'  => '<h2 class="widget-title">',
        'after_title'   => '</h2>',
    ) );