Adwaita: Fix ineffective theming of switch slider

Adwaita: Fix ineffective theming of switch slider

in a specific case, which was applying .slider as a class on the parent
switch, instead of correctly selecting on its child node named slider.

This makes the border on the outside of a switch in a selected listbox
row look better in the light variant. Since the code was never removed,
it was clearly meant to work, and making it work is a clear improvement.