This summary is based on (and quotes) the Qubes Community guide, this and that — especially @Sven’s extensive work, see also this thread.
Dom0, Fedora & Debian
The following text and screenshots describe how to change the default light theme to a global dark theme. It’s just an example; feel free to adjust the appearance to your taste.
And here are the final results:
Dom0 Qube Manager, Dom0 Terminal, Fedora 35 VMs, Debian 11 VM; desktop background color #3B3941 (external full HD display)
Qubes Applications Menu is darkened as well
Note: Of course, several apps (e.g., Terminal and Gedit) can be styled in the app itself. Here, Dom0 Terminal’s background color is set to the desktop background color — because fusion. 
Dom0
Change Appearance
-
Open the
Appearancewindow: -
Qubes Applications Menu →
System Tools→Appearance
-
Select
Style→Adwaita-dark. It’s probably the most ›classic‹ and readable/contrasty of the dark styles here. But, of course, feel free … -
Adjust
Icons,FontsandSettingsto your taste. In all these examples here, icons are always set toGnomeand fonts toDejaVu Sansfamily.
Change Style in Window Manager
-
Open the
Window Managerdialog:Qubes Applications Menu →
System Tools→Window Manager→Style
-
Change
ThemeinStyletab toG2,Wallis, orBluebird. These are probably the most ›classic‹ and readable/contrasty.Bluebird's borders appear more elegant, but it could be difficult to grab the window’s edges in order to resize. But feel free …
Qubes tools
Most of the Qubes OS UI in dom0 is now Qt5 based and doesn’t adhere to the selected theme by default.
more info: Uniform look for Qt and GTK applications - ArchWiki
This can be remedied by:
- using a native Qt style
sudo qubes-dom0-update adwaita-qt5
Then add QT_STYLE_OVERRIDE=adwaita-dark to /etc/environment.
Reboot.
- using a theme engines
sudo qubes-dom0-update qt5-qtstyleplugins
Then add QT_QPA_PLATFORMTHEME=gtk2 to /etc/environment.
Reboot.
- add a menu option to qube manager
That avoid to install anything in dom0,
but it require to review the few lines of code being added.
qube-manager dark mode (built-in method)
Fedora 34 / 35
Install and use Gnome Tweaks
In templateVM:
sudo dnf install gnome-tweaks
Note: See How to update.
-
Shutdown templateVM and add
TweakstoApp shortcutsof appVM in Qube Manager. -
Start
Tweaksin appVM. -
Select
Appearance→Themes→Applications→Adwaita-dark(adjust also all the other options to your taste):
Notes: You have to do this in the templateVM and all appVMs; changes you make in the home directory of a templateVM will not reflect in the template based VM by design. For dispVMs see this.
If you want to use gnome-settings-daemon and Gnome Tweaks, it’s probably best to delete settings.ini and .gtkrc-2.0 (see following Debian section).
To make sure QT apps look good, too: In templateVM install qt5-qtstyleplugins, gtk-murrine-engine and gnome-themes-standard. Add QT_QPA_PLATFORMTHEME=gtk2 to /etc/environment.
Debian 10 / 11
In Debian, things are getting a little bit trickier …
AppVMs seem to work best without gnome-settings-daemon (see end of Fedora section above). In that case set DPI in /etc/X11/Xresources/x11-common using Xft.dpi: (probably 96) in template VM.
Make sure your appVMs have both ~/.config/gtk-3.0/settings.ini and ~/.gtkrc-2.0:
settings.ini
[Settings]
gtk-application-prefer-dark-theme=1
gtk-font-name=DejaVu Sans Book 12
gtk-theme-name=Adwaita-dark
gtk-icon-theme-name=gnome
gtk-application-prefer-dark-theme=1 is not necessary, it’s forces usage of dark variation of selected theme. Since we are choosing theme that is already Dark it can be omitted.
.gtkrc-2.0
include "/usr/share/themes/Adwaita-dark/gtk-2.0/gtkrc"
style "user-font"
{
font_name="DejaVu Sans Book"
}
widget_class "*" style "user-font"
gtk-font-name="DejaVu Sans Book 12"
gtk-theme-name="Adwaita-dark"
gtk-icon-theme-name="gnome"
Note: The icon theme name is the name of its directory, not the name property in its index.theme .
According to: https://wiki.archlinux.org/title/GTK
Put these files to /etc/skel of the templateVM, so they get created automatically if you set up a new appVM based on that templateVM.
To make sure QT apps look good, too: In templateVM install qt5-style-plugins, gtk2-engines-murrine, and gnome-themes-standard. Then add these two lines to /etc/environment:
QT_QPA_PLATFORMTHEME=gtk2
QT_SCALE_FACTOR=1
This work for debian-11-minimal but for debian-11 You have to uninstall xsettingsd or edit ~/xsettingsd config file instead of using ~/.config/gtk-3.0/settings.ini and ~/.gtkrc-2.0.
Done!
To do? Darken Firefox websites and Thunderbird emails just with userChrome.css and userContent.css, not via extensions/add-ons.
Note: Some panel (applet) icons, e.g., NetworkManager — see ›global‹ screenshot at the beginning of this guide —, notification-daemon, Joplin and Keybase, remain with white instead of dark resp. transparent backgrounds. For NetworkManager icon, see Qubes issue #2846.
. Since Yes I installed it but then I reinstall Qubes and try to restore all VMs from backups. But it faild to restore templates and many VMs. But I was able to restore personal qube that is the VM in which I now tried to enable dark mode so it might be some leftovers there. In template I’m not installing gnome-settings-daemon after reinstall.