Hello people ! I've had some hardware problems so I don't use Excalibur anymore right now. I'll reinstall it as soon as I can and make the suggested changes. Thank you again for your help, it's precious !

@rbit : Thank you !

I tried but it didn't work. I had to make a mistake because when I restarted, the screen was black. And I didn't know what to do (I'm a beginner), I had to reinstall Excalibur and it takes me some time. So I'm gonna wait a little while before I make any other changes. Curve-Darkpurpy on login screen works for me right now (It's dark so it's cool for my eyes).

My request is somehow partially resolved. I'll try to come it back later. Thanks to everyone for your help

@brocashelm : Thanks a lot ! I did the sudo nano /etc/slim.conf command, changed the theme, saved and it works ! I'm happy

Then if I chose a personal image instead of a preinstalled theme as background, what command should I do ?

I know there's a way to do it with LightDM but I don't want to install it.

@rbit : Thanks for the answer but I don't know how to modify the files (Devuan and  Gnu-Linux are new things for me).

And when I try to edit /etc/slim.conf, I can't save because the file is in "read-only mode".