bcworkz on "Admin Dashboard Widgets"

The line is done with a CSS directive, something like this:

#plugin_info {
    border-bottom: 1px solid #eee;
}

The background CSS is like this:

#plugin_news {
    background-color: #fafafa;
)

You can apply the CSS with an inline <style> block or as an element style attribute. If you prefer external style sheets, enqueue you CSS file with wp_enqueue_style() called from within a callback function added to the 'admin_init' action hook.