ILUXA said it all
; however enabling autologin it's not the safest of the choices, so I rather just uncommenting the
# default_user username
string, and then uncomment
#focus_password yes
. This writes your username as default in the user_field on the Slim Screen, and focus the cursor on the password field, so as you just have to quickly type your password before entering Xorg.
I'll upload (on my files.fm account) some FreeBSD slim-themes I gathered online, all tested and working well. It's really easy to edit each of them and create your custom one. Download link:
FreeBSD_slim-themes.tar.gz
Also give a look to another lightweight display manager
x11/sddm, and to the nice-looking
lebarondemerde's freebsd SDDM theme:
x11-themes/sddm-freebsd-black-theme.
XDM is obviously the lightest, but it's highly customizeable if you know how