My tilemap includes a slanted tile that has transparency, but Unity gives it a background for some reason and I just couldn't figure out how to change that.
This is how it looks like in the tileset file (opened with GIMP)
Game view (it doesn't work with a background for the game itself either)
I found the solution myself after all! I had to change the tilemap's material. Selected material was Default-Line, but apparently it needs to be Sprites-Default