How to reference nested classes with css?

Answer 1

You just need to know how css selectors work. Here is brief description about css selectors.

In your case,

.box .box-body i{
  width:30px;
}

space between two selectors defines second element is child of first. In your case, element i is child element of element which has box-body class. and that element is child element of class which has .box class.