Custom CSS Classes for WordPress Widgets

plugin - https://wordpress.org/plugins/widget-css-classes/

or
add this to functions.php
//http://ednailor.com/2011/01/24/adding-custom-css-classes-to-sidebar-widgets/
//Custom CSS Classes for WordPress Widgets

function kc_widget_form_extend( $instance, $widget ) {
if ( !isset($instance['classes']) )
$instance['classes'] = null;





/* Set your predetermied class choices here */
$myarray = "class1,class2,class3";

$myclasses = explode(",",$myarray);
$row = "<p>\n";
$row .= "\t<label for='widget-{$widget->id_base}-{$widget->number}-classes'>Class:</label>\n";
$row .= "\t<select name='widget-{$widget->id_base}[{$widget->number}][classes]' id='widget-{$widget->id_base}-{$widget->number}-classes' class='widefat'>\n";
foreach($myclasses as $myclass) {
$instance_selected = null; if($instance['classes']==$myclass) $instance_selected = " selected='selected'";
$row .= "\t<option value='".$myclass."'".$instance_selected.">".$myclass."</option>\n";
}
$row .= "</select>\n";








//Allow the user to assign their own classes:
/* Allows User to Add Custom CSS Classes */
$row .= "\t<input type='text' name='widget-{$widget->id_base}[{$widget->number}][classes]' id='widget-{$widget->id_base}-{$widget->number}-classes' class='widefat' value='{$instance['classes']}'/>\n";
$row .= "</p>\n";







echo $row;
return $instance;
}
add_filter('widget_form_callback', 'kc_widget_form_extend', 10, 2);function kc_widget_update( $instance, $new_instance ) {
$instance['classes'] = $new_instance['classes'];
return $instance;
}
add_filter( 'widget_update_callback', 'kc_widget_update', 10, 2 );
function kc_dynamic_sidebar_params( $params ) {
global $wp_registered_widgets;
$widget_id    = $params[0]['widget_id'];
$widget_obj    = $wp_registered_widgets[$widget_id];
$widget_opt    = get_option($widget_obj['callback'][0]->option_name);
$widget_num    = $widget_obj['params'][0]['number'];

if ( isset($widget_opt[$widget_num]['classes']) && !empty($widget_opt[$widget_num]['classes']) )
$params[0]['before_widget'] = preg_replace( '/class="/', "class=\"{$widget_opt[$widget_num]['classes']} ", $params[0]['before_widget'], 1 );

return $params;
}
add_filter( 'dynamic_sidebar_params', 'kc_dynamic_sidebar_params' );