Improve the look of ON switch

The through is now flatter, and the slider's border is now transparent black instead of gray, so it doesn't look odd on blue background.

https://bugzilla.gnome.org/show_bug.cgi?id=729558
There were unused hover states for checkboxes and radios in the assets - use them

https://bugzilla.gnome.org/show_bug.cgi?id=729561