I had the same problem.
#apt-get install syslinux-themes-debian
wasn't usefull for me, but I succeeded in installing the package directly with dpkg:
# dpkg -i /var/cache/apt/archives/syslinux-themes-debian_12-3_all.deb
I had the same problem.
#apt-get install syslinux- themes- debian
wasn't usefull for me, but I succeeded in installing the package directly with dpkg:
# dpkg -i /var/cache/ apt/archives/ syslinux- themes- debian_ 12-3_all. deb