Manage users custom column add class "num"

I have customized my admin users column to show the purchased leads of a role "lead buyer". The value of the column is a decimal (the number of leads).

The "posts" column of the users table has a class "num" so the <th> and the <td> are styled with text-align: center;.

I want to add a the "num" class to my custom column too.

Does anyone know in which document you can find the "manage_users_columns" and "manage_users_custom_columns" functions to see if there is a possibility to add the class "num"?

// Add users table header columns
add_filter( 'manage_users_columns', 'gtp_users_table_columns' );
function gtp_users_table_columns( $defaults ) {
    $defaults['purchased-leads'] = __( 'Purchased leads', 'gtp_translate' );
    return $defaults;
}

// Add users table lead purchase column content
add_action( 'manage_users_custom_column', 'gtp_users_table_content', 10, 3 );
function gtp_users_table_content( $value, $column_name, $user_id ) {
    $leads = gtp_get_leads_by_buyer( $user_id );
    switch( $column_name ) {
        case 'purchased-leads' : 
            return $leads->found_posts;
            break;

    }
}