This is Qubes 4.1 fully darkened on an external full HD display. \ö/
Thank you — and all the commentators here and here — so much for your hard work! (For me, now, it was just a couple of hours. 
)
Summary for all users
- dom0 darkened with ›Q‹ panel click-pop-up/System Tools/Appearance and /Window Manager. With Style ›Bluebird‹ and in dom0 Terminal setting the same background color as desktop, dom0 Terminal kind of floats on the desktop, I like it …
- Fedora 34 VMs (as ›browser‹, orange, in the screenshot) darkened with gnome-tweak-tools, »Tweaks« in dom0 applications list.
- Debian 11 VMs (as ›encryption‹, purple, in the screenshot) with /etc/environment, settings.ini and gtkrc-2.0 files/settings.
- Background colors and fonts of Gedit and Terminal are, of course, adjustable within the apps itselves.
[So, fortunately, you/we all can avoid a nightmare called KDE (I know what I’m talking about).]
Have a nice one!
[Edit: See new user guide.]