Adwaita: dark: Make colorswatch.light tick visible

Adwaita: dark: Make colorswatch.light tick visible

It used $text_color unconditionally, but in :dark, text is white, so we
overlaid a white tick on any light colours, all the way to white itself.

Using these named colours doesn’t make practical or semantic sense.
Instead, use white/black over dark/light swatches, as in HC, so all
variant–swatch combos work. Light looks the same, & :dark works now.

For backdrop, use alpha 0.5, unlike 0.7 in HC, as that seemed excessive
& different from the current effect. 0.5 is almost identical to how
$backdrop_fg_colour is a 50% mix of $fg_color, & matches backdrop text.

https://bugzilla.gnome.org/show_bug.cgi?id=787531