themes: Make fallback work a bit better

One case we did not include in fd329e0e is

Adwaita-dark -> Default:dark

It turns out that it is needed.
12 jobs for theme-fallback-fix in 21 minutes and 8 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  .Pre
passed #1255702
style-check-diff

00:03:05

 
  Build
passed #1255703
fedora-x86_64

00:08:34

passed #1255705
installed-tests

00:07:29

passed #1255707
macos
macos

00:11:24

passed #1255706
win32-ps
msys2-mingw64

00:05:38

passed #1255704
release-build

00:03:47

 
  Analysis
failed #1255709
asan allowed to fail
asan-build

00:21:05

passed #1255708
static-scan

00:09:55

 
  Docs
passed #1255710
reference

00:07:23

 
  Flatpak
manual #1255711
flatpak allowed to fail manual
flatpak-manual:demo
manual #1255713
flatpak allowed to fail manual
flatpak-manual:icon-browser
manual #1255712
flatpak allowed to fail manual
flatpak-manual:widget-factory
 
Name Stage Failure
failed
asan-build Analysis
 - css: 214 total, 214 pass, 0 fail
Processing gtk suite a11y:
- a11y: 22 total, 22 pass, 0 fail
Processing gtk suite tools:
- tools: 4 total, 3 pass, 1 fail
Processing gtk suite reftest:
- reftest: 132 total, 131 pass, 1 fail
Cleaning up file based variables
ERROR: Job failed: exit code 1