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.
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