Hello Radomír Polách !

You may add the new rules for .sys_menu_item to the media CSS files in your template, like media-desktop.css, media-phone.css, media-print.css and media-tablet.css.

If you use Protean template then you should copy them from template/css folder to modules/boonex/protean/data/template/system/css and edit them with new rules.