Adwaita: Fix color of focus outline on colorswatch

Adwaita: Fix color of focus outline on colorswatch

The focus outline disappeared as the colour of the swatch got close to
the normal focus outline colour, which is alpha(currentColor, 0.3).

Fix by making the outline an alpha’d version of the tick colour, but
more opaque than normal outlines. 0.6 seems good enough; feel free to
improve it, but at least this ensures the outline can’t vanish anymore.

HighContrast achieves this already because it applies the color property
to the main node, not the overlay. Doing that means the outline is fully
opaque, which is fine for HC obviously but was excessive for Adwaita.

https://bugzilla.gnome.org/show_bug.cgi?id=787757