Wanted to add my watermark files to Darktable 2.6 in Mint but I installed if with a flatpack. For the life of me I can not figure out where is located so I can copy the files over.
Move the mouse in the watermark module over the ‘marker’ menu, paths should be shown in the tooltip.
~/.var/app/org.darktable.Darktable/config/darktable I think… That is off the top of my head 
Thanks, found it.