Using a CSS 3 selector in a browser version that doesn’t support it yet? The only browser I can find that supports it is Opera. Neither Firefox or Chrome recognise those selectors.
Also you have the wrong sort of " around the word checkbox.
It’s actually working in all the browsers that we listed in the reference. (assuming you use the correct quote marks)
The problem is that you can’t change the background colour of a check box in those browsers (you can change the background in IE but it doesn’t support the :checked property)