gtk3, gtk4: Install gtk-dark.css for Arc-Dark

With the new "dark style preference" introduced with GNOME 42, when
dark style is requested while using a theme that doesn't have a
gtk-dark.css, GTK will use the default dark theme Adwaita-Dark,
instead of falling back to just using the non-dark version of the
selected theme, which was the old behavior when applications
requested dark theme. Fix this situation in Arc-Dark by adding the
gtk-dark.css files just like with other variants.

Fixes: #180

common/gtk-3.0/meson.build

@@ -136,16 +136,14 @@ foreach variant : get_option('variants')
  rename : ['gtk.css']
  )
 
- if variant != 'dark'
- #FIXME gtk-dark.css gets overwritten for subsequent variants, resulting in build warnings
- gtk3_dark_css = configure_file(
- capture : true,
- command : ['echo', '@import url("resource:https:///org/gnome/arc-theme/' + output_css_dark + '");'],
- output : 'gtk-dark.css',
- install : true,
- install_dir : prefix / install_dir.get(variant) / common_dirs.get('gtk3')
- )
- endif
+ #FIXME gtk-dark.css gets overwritten for subsequent variants, resulting in build warnings
+ gtk3_dark_css = configure_file(
+ capture : true,
+ command : ['echo', '@import url("resource:https:///org/gnome/arc-theme/' + output_css_dark + '");'],
+ output : 'gtk-dark.css',
+ install : true,
+ install_dir : prefix / install_dir.get(variant) / common_dirs.get('gtk3')
+ )
 
  # install thumbnail
  install_data(

common/gtk-4.0/meson.build

@@ -136,14 +136,12 @@ foreach variant : get_option('variants')
  rename : ['gtk.css']
  )
 
- if variant != 'dark'
- #FIXME gtk-dark.css gets overwritten for subsequent variants, resulting in build warnings
- gtk4_dark_css = configure_file(
- capture : true,
- command : ['echo', '@import url("resource:https:///org/gnome/arc-theme/' + output_css_dark + '");'],
- output : 'gtk-dark.css',
- install : true,
- install_dir : prefix / install_dir.get(variant) / common_dirs.get('gtk4')
- )
- endif
+ #FIXME gtk-dark.css gets overwritten for subsequent variants, resulting in build warnings
+ gtk4_dark_css = configure_file(
+ capture : true,
+ command : ['echo', '@import url("resource:https:///org/gnome/arc-theme/' + output_css_dark + '");'],
+ output : 'gtk-dark.css',
+ install : true,
+ install_dir : prefix / install_dir.get(variant) / common_dirs.get('gtk4')
+ )
 endforeach