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)
Status Job ID Name Coverage
  Build
passed #267142
librem5
api-visibility

00:02:36

passed #267144
librem5
build-debian-clang

00:04:24

passed #267143
librem5
build-debian-gcc

00:03:00

passed #267145
librem5
build-fedora-gcc

00:03:39

 
  Test+Docs
passed #267146
librem5
build-gtkdoc

00:02:21

passed #267147
librem5
unit-test

00:02:18

41.1%
 
  Package
passed #267148
librem5
package-debian-buster

00:02:05

passed #267149
librem5:arm64
package-debian-buster:arm64

00:05:59

failed #267150
flatpak allowed to fail
package-flatpak

00:00:14

failed #267151
flatpak allowed to fail
package-flatpak-glade

00:00:14

 
  Test Package
passed #267152
librem5
autopkgtest-debian-buster-package

00:02:00

passed #267153
librem5
lintian-debian-buster-package

00:00:46

 
Name Stage Failure
failed
package-flatpak-glade Package
rm: cannot remove '_build/meson-private/cmd_line.txt': Permission denied
rm: cannot remove '_build/meson-private/cleantrees.dat': Permission denied
rm: cannot remove '_build/meson-private/meson_exe_glib-mkenums_c80cfe2273ab2e4bbde
Running after_script
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=267151 responseStatus=201 Created token=fKGCFKj_
ERROR: Job failed: exit code 1
failed
package-flatpak Package
rm: cannot remove '_build/glade/fe02414@@glade-handy@sha/glade-hdy-preferences-window.c.gcno': Permission denied
rm: cannot remove '_build/glade/fe02414@@glade-handy@sha/glade-hdy-header-bar.c.o': Permission denied
rm: cannot remove '_build/glade/libglade-handy.so': Permission denied
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=267150 responseStatus=201 Created token=SS1_LgQZ
ERROR: Job failed: exit code 1