High-contrast border on overlays

[GeopJr]

Forwarding from https://github.com/GeopJr/Tuba/pull/1428

I added some styles for HC, specifically the typical HC border around the controls (box-shadow: inset 0 0 0 1px var(--border-color);) and made the background opaque.

I doubt it's possible for clapper-gtk*, but it can be done at least in clapper-app

*because hc is only on libadwaita/handy afaik