themes: Simplify shadows definition

Let's use the power of SASS to simplify definitions. This also fixes an
upward shadow definition error.
11 jobs for theming-overhaul in 14 minutes and 50 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #254398
librem5
api-visibility

00:02:46

passed #254400
librem5
build-debian-clang

00:04:01

passed #254399
librem5
build-debian-gcc

00:03:00

passed #254401
librem5
build-fedora-gcc

00:04:04

 
  Test+Docs
passed #254402
librem5
build-gtkdoc

00:02:25

passed #254403
librem5
unit-test

00:02:21

42.0%
 
  Package
passed #254404
librem5
package-debian-buster

00:02:26

passed #254405
librem5:arm64
package-debian-buster:arm64

00:05:55

passed #254406
flatpak
package-flatpak

00:01:46

 
  Test Package
passed #254407
librem5
autopkgtest-debian-buster-package

00:02:24

passed #254408
librem5
lintian-debian-buster-package

00:00:57