After migrating sites, the above link will not work.
Here's the relevant bit:
Somewhere in your functions.php your theme will make use of register_sidebar. It should look something like:
'name' => __( 'Side bar name' ),
'id' => 'sidebar-id',
'description' => __( 'Widget area description' ),
'before_title' => '<h1>',
'after_title' => '</h1>',
'before_widget' => '<div id="%1$s" class="widget %2$s">',
'after_widget' => '</div>',
Notice the %1$s and %2$s in the before_widget argument. These allow the widget to add their own ID and classes to the widget. It seems your theme does not use class="widget %2$s". If you add this in – that should fix the problem. You may want to contact the theme author and alert them to this problem – themes should allow widgets to add their own classes and ID.