I am able to write to the /usr/local directory, but not anywhere else outside of it - existing forum posts have confirmed that this is indeed possible. GDM is supposed to detect files from the XDG_DATA_DIRS environment variable, but it’s not doing so.
I did see Kinoite, but that’s only for common desktop environments… I’m interested in using a window manager that is neither of the options included in that thread. I’ve looked around for implementation details of how Kinoite was made and haven’t found anything - only instructions on how to use it exist. Such implementation details would be more useful than a premade package so you can use any desktop environment you desire.
And yes, this .desktop file exists in /var/usrlocal/share/…