demo: Add a dark theme toggle button

This will help testing the dark theme mode.

Fixes Librem5/libhandy#233.
11 jobs for dark-theme in 17 minutes and 54 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #255903
librem5
api-visibility

00:02:44

passed #255905
librem5
build-debian-clang

00:03:50

passed #255904
librem5
build-debian-gcc

00:02:58

passed #255906
librem5
build-fedora-gcc

00:05:25

 
  Test+Docs
passed #255907
librem5
build-gtkdoc

00:03:03

passed #255908
librem5
unit-test

00:03:07

43.7%
 
  Package
passed #255909
librem5
package-debian-buster

00:03:25

passed #255910
librem5:arm64
package-debian-buster:arm64

00:06:23

passed #255911
flatpak
package-flatpak

00:01:41

 
  Test Package
passed #255912
librem5
autopkgtest-debian-buster-package

00:02:53

passed #255913
librem5
lintian-debian-buster-package

00:01:04