The H2O nightmode is working for me as well. After setting it to 3% (it's kind of clumsy to nail it to the exact percent), I had to close the brightness window, holding the brightness icon again and voilą!, it changed to negative picture. I Had to repeat the exact same process to switch it back so once you know how to do it it's quite useful.
I don't understand why Kobo doesn't have this feature by default, my android reading app had this nice feature included and it was much more pleasant to read in total darkness.
Thank you everyone, I can finally read in the dark without destroying my eyes
