quick CSS for current_page_item

Dear Folks,

I use quick CSS to adding style to the widget menu :

#top .widget ul a:hover {
background-color: gold;
padding: 0% 4%;
letter-spacing: 0.007em;
border-radius: 9px;
}

but i don’t know how change style of link for the current page item.
I would like apply a blue background-color for the current page.
Can you help for this ?

Best regards,
James