theme: Adjust leaflet/deck shadow for dark and HC

It's almost invisible in dark variant, so make it more opaque. Use
$border_color for border for HighContrast, as it doesn't have to look
pretty there and the regular one isn't visible enough.

Fix formatting in the background-shadow mixin and use rgba(0, 0, 0, x)
instead of hdyalpha(#000, x) for consistency.
12 jobs for wip/exalm/hc in 18 minutes and 10 seconds (queued for 2 seconds)