All DEs I have tried made it easy to customize themes, as I recall, it is just a headache that I need to spend so much time seeing things up, usually after an install, and in the case of the Pyra, after I get it, before I can get down to business. I know, probably a stupid complaint. I know I am not alone, though, as dark themes have been present for a long time, even on Windows in the 90s.
Anyway, yeah, if Xfce is going to be the default, I would appreciate a decent dark theme be included. It needn't be anything that you spend much time on, just one that you use that ED approves or whatever. If I have used Mate it was a VM or someone else's install, and was rather brief, so not one I am familiar with. What will be the default upon shipping? Regardless of what else I add (KDE possibly) or change (need to declutter the desktop, make things a bit more minimalist in general, move the bar to the right side), I plan to have whatever it ships with around so I can help people troubleshoot. I also expect to put the micro SD slot to use to run other distros, so if go with something other than Debian as my main distro, Debian will remain mostly stock and only really used when I need to boot into it to help others. That is when it will be especially nice not to burn my eyes while trying to help out others. All I am looking for is something that isn't headache inducing which allows me to seeasily see everything...not sure if I am being clear enough.
TLDR, if Xfce is the default DE, yes, please include a decent dark theme.
If I need to contact ED to make an official request, lemme know. I wish this forum had a dark theme, too.