/* CSS comment */

In the middle Screen on top you will see 2 so-called tabs an index.html tab and a stylesheet.css tab, also called the CSS-tab. click on the tab and you can _modify the ‘file’ stylesheet.css which called upon by the index.html file via

 <link type="text/css" rel="stylesheet" href="stylesheet.css">

The CSS facilty uses so-called Selectors to which you attach a pair of parentheses-{ } in-between which you then define CSS-properties.

Selector {
     css-property;
     property-key: property-Value ;
     background-color: blue;
}

As jQuery uses the same Selector-syntax you could look at http://www.w3schools.com/jquery/jquery_ref_selectors.asp to get a feeling of which possible selector-type’s can be used.