theme: Split shared style into shared and fallback

The fact the shared style is loaded at override priority is causing
problems for elementary: Librem5/libhandy#273

While window corners are fixed by the previous commit, they aren't the
only area that they may want to style. Split the part that's just visuals
into a separate stylesheet named fallback.css, and load it at a fallback
priority for all stylesheets, leave shared for critical styles, such as
layout.
12 jobs for wip/exalm/fallback in 16 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #267087
librem5
api-visibility

00:02:34

passed #267089
librem5
build-debian-clang

00:04:37

passed #267088
librem5
build-debian-gcc

00:03:02

passed #267090
librem5
build-fedora-gcc

00:04:06

 
  Test+Docs
passed #267091
librem5
build-gtkdoc

00:02:36

passed #267092
librem5
unit-test

00:02:33

41.1%
 
  Package
passed #267093
librem5
package-debian-buster

00:02:02

passed #267094
librem5:arm64
package-debian-buster:arm64

00:06:06

failed #267095
flatpak allowed to fail
package-flatpak

00:00:15

failed #267096
flatpak allowed to fail
package-flatpak-glade

00:00:15

 
  Test Package
passed #267097
librem5
autopkgtest-debian-buster-package

00:02:56

passed #267098
librem5
lintian-debian-buster-package

00:01:11

 
Name Stage Failure
failed
package-flatpak-glade Package
rm: cannot remove '_build/src/25a6634@@handy-1@sha/hdy-leaflet.c.gcda': Permission denied
rm: cannot remove '_build/src/25a6634@@handy-1@sha/hdy-header-group.c.gcda': Permission denied
rm: cannot remove '_build/src/25a6634@@handy-1@sha/hdy-navigation-direction.c.
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=267096 responseStatus=201 Created token=Db9Wx9xP
ERROR: Job failed: exit code 1
failed
package-flatpak Package
: Permission denied
rm: cannot remove '_build/src/25a6634@@handy-1@sha/hdy-title-bar.c.gcda': Permission denied
rm: cannot remove '_build/src/25a6634@@handy-1@sha/hdy-keypad-button.c.o': Permission denied
rm:
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=267095 responseStatus=201 Created token=voWhxfxd
ERROR: Job failed: exit code 1