Crea un bash script llamado startdwm:
$ cat /usr/bin/startdwm
while true ; do xsetroot -name "$(cat /sys/class/thermal/thermal_zone4/temp) | `date`"; sleep 2
0; done &
keynav &
brightnessctl s 30% &
mbsync -a &
st -e neomutt &
exec dwm
Guardalo en /usr/bin
Ahora andá a /usr/share/xsessions/, copiá una existente y pone Exec=startdwm o podés usar esta:
$ cat /usr/share/xsessions/dwm.desktop
[Desktop Entry]
Version=1.0
Name=DWM
Comment=Use this session to run DWM as your desktop environment
Exec=startdwm
Icon=
Type=Application
Esto es todo