Thanks for the tip. The "lsmod | grep efi" command yielded empty output in my existing distributions, but it gave me some output when I ran sparky live. So my motherboard does support efi. But I did not want to use efi.
So the tip did not really work, but I think I have figured out how to install sparky in non-efi mode.
First of all, the motherboard did support efi mode ; the problem was that my other existing distributions had been installed in legacy mode, so that I needed to install Sparky in legacy mode. Which turns out to be difficult to do.
The "advanced installer" seems not to allow legacy installations. Although the installer says that I can make the choice of efi or non-efi mode for installation, it does not actually give me that choice.
Similarly the regular installation also does not give me the choice.
The only way seems to be to use the regular installer, in "expert" mode. So I mounted the partition to which I wanted to install sparky to "/target", and went through the "expert" installation, which seems to have worked.