stylesheet: Overhaul checks and radios

Dark+checked is too dark atm, but it's another instance of #171 (closed), and this is one of the steps that will allow to lighten it and have this:

Menu:

Unfortunately there are some regressions with how checks are shown in GtkTreeView, but that seems to be an unrelated bug given we already had stuff like this:

(yes, that's a radio, but with border-radius of a check)