The ugly transition between LigthDM, or more specifically its GTK greeter, and Openbox is kind of annoying. A gray background pops up shortly before the desktop wallpaper gets drawn (assuming you use a wallpaper).

The ugly transition in action:

Digging around, I found out that Openbox actually runs x/e/hsetroot at startup. to set the background color. To get rid of that ugly, temporary background during logging in, all you have to do is either delete or comment out the lines in Openbox's autostart bash script.

:::bash
# Set a background color
#BG=""
#if which hsetroot >/dev/null 2>/dev/null; then
#  BG=hsetroot
#elif which esetroot >/dev/null 2>/dev/null; then
#  BG=esetroot
#elif which xsetroot >/dev/null 2>/dev/null; then
#  BG=xsetroot
#fi
#test -z $BG || $BG -solid "#303030"

On Arch, the file you have to edit is /usr/lib/openbox/openbox-autostart. Other distributions probably keep it in the same place, but you can find the autostart's file location by checking the output of

ps x | grep openbox

Finally, the transition should be smooth now, especially if your display manager's background is the same as your desktop background.