Observably I were referring to the image in question and you'd use the 'class', or 'id' if there was the necessity, or apply the CSS "Attribute Selectors" which as far as I can recall fail in Micro$oft IE.
Actually you'd get some quite interesting results if you added width and height dimensions to a checkbox, well you certainly do in the early Mozilla since the checkbox and radio button would grow to those dimensions, thus in practice you can end-up with giant checkboxes, have fun:
As Redux implied radio buttons can be finicky beasts to style, the last time I began messing with radio buttons and colour states via CSS was a couple of years back, and I totally agree the results are not always what you expect ...
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
background-color : gray;
width : 100px;
height : 100px;
<form action="#" method="post" id="myForm">
<input type="radio" name="choice" />
<input type="checkbox" value="submit" />
<input type="submit" value="submit" />
<input type="reset" value="reset" />