Debian homeworld theme
I see homeworld will be the theme for Debian 11. That is, Debian bullseye is the current testing distribution and it will become Debian 11 sometime next year or so. When it does, the default backgrounds and colors will be this gorgeous theme by Juliette Taka.

Keen!
I've been running bullseye on my laptop for a while now, but it still has the Debian 10 background for a login screen. I changed the background once I log in, but I wanted to try these homeworld backgrounds for both login and wallpaper. I am running LightDM and Xfce, so here is what I did.
Clone Juliette Taka's homeworld repo.
git clone https://github.com/julietteTaka/homeworld.git
Make symbolic links (or just copy) the images to desktop-base
.
cd /usr/share/images/desktop-base
sudo ln -s ~/homeworld/Desktop\ Wallpaper/homeworld_wallpaper_2560x1600.svg homeworld.svg
sudo ln -s ~/homeworld/Login\ Screen/homeworld_login_2560x1600.svg homeworld-login.svg
Edit the lightdm config to use the new image. (I have E as an alias to sudoedit for such things. alias E="EDITOR=\"emacsclient -c -a emacs\" sudoedit"
)
E /usr/share/lightdm/lightdm-gtk-greeter.conf.d/01_debian.conf
Mine now looks like this.
$ cat /usr/share/lightdm/lightdm-gtk-greeter.conf.d/01_debian.conf
[greeter]
background=/usr/share/images/desktop-base/homeworld-login.svg
theme-name=Homeworld
xft-antialias=true
xft-hintstyle=hintfull
xft-rgba=rgb
reader=orca
Now I have a new login screen!

I just right-clicked on the background and chose "Desktop Settings…" and then selected the homeworld wallpaper. That will only be for my user, but I'm the only one on this machine. I'm not sure what to do to make the homeworld wallpaper the default. Presumably, that will be the case when Debian 11 is released!

Thanks to Juliette Taka and everyone at Debian!