I haven’t seen a fix for it. I opted to install KDE using this guide: KDE - changing the way you use Qubes - #8 by Sven
From there, the dark theme is trivial to implement. I also use debian as my default template for my appVMs. That requires a template install of gnome-tweak-tool and a few simple changes to ~/.config in corresponding appVMs to have dark themes both system wide and in your VMs.