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.
13 jobs for master in 14 minutes and 59 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Build
passed #267154
librem5
api-visibility

00:02:33

passed #267156
librem5
build-debian-clang

00:04:01

passed #267155
librem5
build-debian-gcc

00:02:57

passed #267157
librem5
build-fedora-gcc

00:03:45

 
  Test+Docs
passed #267158
librem5
build-gtkdoc

00:02:24

passed #267159
librem5
unit-test

00:02:26

41.1%
 
  Package
passed #267160
librem5
package-debian-buster

00:02:03

passed #267161
librem5:arm64
package-debian-buster:arm64

00:05:59

failed #267162
flatpak allowed to fail
package-flatpak

00:00:14

failed #267163
flatpak allowed to fail
package-flatpak-glade

00:00:14

 
  Test Package
passed #267164
librem5
autopkgtest-debian-buster-package

00:02:05

passed #267165
librem5
lintian-debian-buster-package

00:00:51

 
  Deploy
passed #267166
librem5
publish-docs

00:00:24

 
Name Stage Failure
failed
package-flatpak-glade Package
rm: cannot remove '_build/src/25a6634@@handy-1@sha/hdy-action-row.c.gcno': Permission denied
rm: cannot remove '_build/src/25a6634@@handy-1@sha/hdy-deck.c.o': Permission denied
rm: cannot remove '_build/src/25a6634@@handy-1@sha/hdy-header-group.c.gcno': Permission denied
Uploading artifacts for failed job
Uploading artifacts...
_build/meson-logs/meson-log.txt: found 1 matching files

WARNING: sm.puri.Handy.Glade-dev.flatpak: no matching files
Uploading artifacts to coordinator... ok
id=267163 responseStatus=201 Created token=2Dsbcmy6
ERROR: Job failed: exit code 1
failed
package-flatpak Package
rm: cannot remove '_build/src/Handy-1.typelib': Permission denied
rm: cannot remove '_build/src/hdy-enums.c': Permission denied
Running after_script
Uploading artifacts for failed job
Uploading artifacts...
_build/meson-logs/meson-log.txt: found 1 matching files

WARNING: sm.puri.Handy.Demo-dev.flatpak: no matching files
Uploading artifacts to coordinator... ok
id=267162 responseStatus=201 Created token=nC1GfBSX
ERROR: Job failed: exit code 1