I have a select
box and I\'m trying to change the background color of the options when the select
box has been clicked and shows all the options.>
If you really want to style the within a , consider switching to a Javascript/CSS based drop down such as http://getbootstrap.com/2.3.2/components.html#dropdowns or https://silviomoreto.github.io/bootstrap-select/examples/. This because browsers such as IE do not allow styling of options within elements. Chrome/OSX also has this problem - you cannot style options.
However a warning is attached to that approach. These types of menus work very differently on mobile platforms because native elements aren't used. They can have annoying quirks on desktop as well. My advice is 1) don't write your own and 2) find a library that's been really well tested.