Your GNOME applications (e.g. Software Center) shows light, even though you’ve selected a dark theme in XFCE?

Me too.

Stock Xubuntu 24.04 had this issue for my wife. She prefers the Software Center over Synaptic, because they don’t show a confusingly long list of packages.

Updates

• @smcv responded on the #1358 issue. Updated the suggested file name accordingly.
• I filed Ubuntu bug 2076999.

Summary

• Check the color-scheme key in GSettings.
• Add an xfce-portals.conf that forces the gtk implementation.
  • This file is installed by default with xfce4-session at 4.18.4 or newer, but Ubuntu 24.04 is at 4.18.3. Debian Testing is at 4.18.4.

[preferred]
default=gtk;

GSettings

First, I checked the obvious things, the theme setting:

$ gsettings list-recursively | grep theme
org.gnome.desktop.interface cursor-theme 'Adwaita'
org.gnome.desktop.interface gtk-key-theme 'Default'
org.gnome.desktop.interface gtk-theme 'Greybird-dark'
org.gnome.desktop.interface icon-theme 'ubuntu-mono-dark'
org.gnome.desktop.sound theme-name 'freedesktop'
org.gnome.desktop.wm.preferences theme 'Adwaita'

Adwaita is the default theme, and is light. Changing them made no difference. Seems the gtk-theme key is what matters.

Next, (color) schemes:

$ gsettings list-recursively | grep scheme
org.gnome.desktop.interface color-scheme 'prefer-dark'
org.gnome.desktop.interface gtk-color-scheme ''
org.gnome.shell.ubuntu color-scheme 'prefer-dark'

The gtk-color-scheme is supposed to be a list of colors, so that should probably be empty. Everything also seems right.

XDG Desktop Portal

Flatpak has introduced something called the Desktop Portal, which is an abstraction over different desktop environments, for package managers to use. Among other things, you can read the color scheme from settings. It turns out the newest GNOME UI library, libadwaita, prefers to use this source over gsettings. Ok, so what’s that doing?

The portal is split into a main process that is a proxy, deciding which backend to use. We’re interested in the GTK backend, since XFCE is GTK-based.

After some digging through StackOverflow and documentation:

$ dbus-send --print-reply --session \
    --dest=org.freedesktop.portal.Desktop \
    /org/freedesktop/portal/desktop \
    org.freedesktop.portal.Settings.ReadAll \
    array:string:\*
Error org.freedesktop.DBus.Error.UnknownMethod: No such interface “org.freedesktop.portal.Settings” on object at path /org/freedesktop/portal/desktop

That can’t be right. I checked the log files. I ran /usr/libexec/xdg-desktop-portal -vr. Nothing stood out.

The GTK backend actually responds correctly:

$ $ dbus-send --print-reply --session \
    --dest=org.freedesktop.impl.portal.desktop.gtk \
    /org/freedesktop/portal/desktop \
    org.freedesktop.impl.portal.Settings.ReadAll \
    array:string:\*
method return time=1723575573.316082 sender=:1.158 -> destination=:1.164 serial=51 reply_serial=2
...

So the problem is that the main process isn’t talking to the GTK backend.

The configuration for the GTK backend actually says:

[portal]
DBusName=org.freedesktop.impl.portal.desktop.gtk
Interfaces=org.freedesktop.impl.portal.FileChooser;org.freedesktop.impl.portal.AppChooser;org.freedesktop.impl.portal.Print;org.freedesktop.impl.portal.Notification;org.freedesktop.impl.portal.Inhibit;org.freedesktop.impl.portal.Access;org.freedesktop.impl.portal.Account;org.freedesktop.impl.portal.Email;org.freedesktop.impl.portal.DynamicLauncher;org.freedesktop.impl.portal.Lockdown;org.freedesktop.impl.portal.Settings;
UseIn=gnome

Which suggests that it matches against the desktop name with the UseIn key. Since we’re on “XFCE”, “gnome” isn’t going to match. Adding XFCE made no difference.

Source Code Digging

After checking the xdg-desktop-portal source code, I noticed that “No skeleton to export” was logged once. So that indicates there are no settings backends being picked up. I already saw that the GTK backend was running, and that the main process was really finding that it supported Settings. But the process never logs “Using gtk.portal for org.freedesktop.impl.portal.Settings (config)”, and that really is the only way a Settings could be added…

As for the UseIn, it’s not used for Settings, unlike all the other interfaces. And it’s deprecated anyway.

So I added the configuration listed above, logged out and in again, and now Software Center is dark!

Discussion

If you search for portals.conf on packages.ubuntu.com, it seems that desktop environments are supposed to have a file they define. The gnome-session-common package contains a file with default=gnome;gtk;, which makes sense. There doesn’t seem to be any for XFCE.

I guess we could just install the gnome-session-common package. But it adds a bunch of systemd service units for the session manager, with dangling command lines (the session manager isn’t installed).

However, I don’t really understand how this is supposed to work on machines where different users use different desktop environments. This is just one global configuration. The UseIn helps with multiple desktop environments, so why is that deprecated?

Why is settings initialized differently from all the other interfaces? Well, there is xdg-desktop-portal#1358 which would make it work the same.

Update: GitHub #1358

@smcv explained that the new backend portals.conf files actually are per-desktop environment, because the prefix (marked as asterisk in the synopsis) actually match against desktop name. He filed Debian bug #1050802 for it a year ago. And it is in xfce4-session since version 4.18.4. Unfortunately, Ubuntu’s xfce4-session is at 4.18.3. Unlucky, I guess.

Debian Testing (Trixie) has 4.18.4. So I filed Ubuntu bug 2076999 to ask for a backport or upgrade.

Conclusion

This is why I’m on open source software. I can solve issues. Sure, it’s sad I had to do it, but all software has bugs and compatibility issues, and I prefer being able to fix it right away.

Admittedly, I could have spent that hour (or two) in a different way. But, sometimes, I rather like spending an hour solving a program mystery, as opposed to just watching a fake mystery being solved on the TV.