alright, I got errors when trying to specify custom config file, so decided to use launch options. These options make it easy to tell separate containers apart.
Arch: alacritty -o window.title=Arch-toolbx -o "colors.primary.background='#314957'" --command distrobox-enter -n arch-toolbx
Ubuntu: alacritty -o window.title=Ubuntu-toolbx -o "colors.primary.background='#404325'" --command distrobox-enter -n ubuntu-toolbx
Fedora: alacritty -o window.title=Fedora-toolbx -o "colors.primary.background='#6C323A'" --command distrobox-enter -n fedora-toolbx