I would like to give my nectar slider image an custom overlay. I have no idea how to do this?

Do i have to add this

.overlay {
  background: none repeat scroll 0 0 rgba(153,255,153,0.2);
min-height: 300px;
}

to the themes custom css and then in the nectar slider image where it says

'Extra Class Name' add 'overlay'?

one more question. When i give the nectar slider an fixed hight for example 430px.

My image will shrink to this size instead i would like it to cut of the overflown and keep the normal size for the 430px and hide the rest. How do i do this?

I have tried the overlay thing didnt work i dont know how to use the short code Extra class name to give the picture i bit of green. It works on a non wordpress html/css site.

Thanks anyway.

Greetings jeroen