Hello Federico,
In most themes the selected items in the MultiSelect are highlighted with a different background color, but in the Material theme (shown in the screenshot) the selected itemsare marked with different text color (blue) and only the focused item is highlighted in gray. You can customize this through CSS as shown in this
dojo example. Note that the first part of the CSS rule's selector:
uses the id of the MultiSelect's dropdown ("products_listbox"), whereas the id of the MultiSelect widget is "products". The dropdown id is generated automatically by appending "_listbox" to the widget's id.
Regards,
Ivan Danchev
Progress Telerik