Stylesheet: update check boxes and radio buttons to match the new style

We're going for a lighter, more open, feel. This is reflected in the borderless buttons and lighter header bars. It can also be seen in the new switches, which have lost their hard borders.

Some controls still have an old style appearance which doesn't match the rest of the updated styling. Check boxes and radio buttons are two prime examples:

They feel cramped and tiny.

I'd suggest making them larger, matching the colours in the switches, and using shadows:

Also, if we did this, we might be able to remove the separate radio button style that we use for ListBoxes.