go for NORD theme
https://github.com/EliverLara/Nordic
and I love this icon set (white)
https://www.xfce-look.org/p/1277095
for more NORD integration have a look here:
https://www.nordtheme.com/ports
have fun
As for the icon theme, Elementary XFCE works perfectly well with Zukitre. If not, ePapirus or Papirus itself. Simple and flat but contrasted, the opposite to a good chunk of flat themes today, where you can't guess where the buttons start and end.
Once you get used to that theme the Night Mode it's useless as I you can just spawn
sct 5500 #or xsct
at daytime, or sct 3500
at night time.xsct/xsct will work with any window manager, too. And the Zukitre themes blend really well with minimal window managers as CWM, i3, DWM and the like, as it has neither curves nor gradients.
I added i3 so everything is on the keyboard.
XFCE is great because it lets you put it in the background. The GUIs are there when you need them, but it is just as happy if you don't.
TBH I typically run things fullscreen, so the only part of xfce I normally "see" is a thin task bar at the bottom with open windows and clock and such. Well, except for when I use Thunar, which is a nice enough file manager.
You're probably not the target audience then. It's not a DE that prioritizes prettiness.
If you want something that looks like the 90's desktop metaphor, it's exactly that and it's really good at that.
For dark mode, try: - in 'Appearance': set Adwaita (dark), - in 'Window Manager': set 'Default', - in 'Panel': set dark mode.
This works in Debian 12 (running XFCE 4.18) and looks beautiful. Easy on the eyes, readable, comfortable.
For other themes look at xfce-look.org. You install these by decompressing tarballs into ~/.themes/$(theme_name) folder and then selecting these in settings manager.
I just looked at the homepage to see if it was anything different than I see on my machine, and if anything it looks nicer there. It's certainly nothing fancy, but I feel like there's hardly enough there to really count as "ugly". It all fades into the background quickly when you're doing actual work on it. But YMMV I guess.
(edit - there are a ton of themes out there: https://www.xfce-look.org
Though personally I would avoid using their app)
Qogir-dark icon theme.
Whisker menu, application icon+ labels and the system tray thing in the bottom panel.
Basically, it looks like windows 7.