I have been testing Debian on my laptop for the last couple days. I am really impressed by it but, of course, testing also means screwing things up. No surprise, I managed to accidentally remove a bunch of useful apps, including the package manager, Synaptic. Yeah, me ;)
Reinstalling was no big deal but I could not get it to use Debian/Xfce's dark them it was using prior its accidental and totally not my fault removal.
Searching for a solution, I learned a few stuff among which Synaptic runs as root (hence the password it requires at launch, who'd have guessed) and that the theme it is using is not directly linked to my own personal theme.
The solution seems to be to link your user's themes to the root's theme, like this:
sudo ln -s ~/.themes /root/.themes sudo ln -s ~/.icons /root/.icons sudo ln -s ~/.fonts /root/.fonts
I have no idea if there is no potential risk in doing so but it does the trick.