fbd-dev-sound: Honour sound theme name for all GNOME based desktops

Phosh recently changed to XDG_CURRENT_DESKTOP=GNOME:Phosh and has since stopped
using the configured theme and instead used the fallback freedesktop theme.

Closes #41
8 jobs for master in 12 minutes and 34 seconds (queued for 2 minutes and 55 seconds)
Status Job ID Name Coverage
  Build
passed #359826
librem5
build:native-debian-bullseye

00:01:38

passed #359824
librem5
build:native-debian-buster:full

00:01:14

passed #359825
librem5
build:native-debian-buster:lib

00:01:08

 
  Test
passed #359827
librem5
test:native-debian-bullseye

00:01:29

62.6%
 
  Package
passed #359828
librem5
package:deb-debian-buster

00:01:58

passed #359829
librem5:arm64
package:deb-debian-buster:arm64

00:05:03

 
  Test Package
passed #359830
librem5
autopkgtest-debian-buster-package

00:01:30

passed #359831
librem5
lintian-debian-buster-package

00:01:27