This is a template for making and modifying simple tilesets. It is 192x192 pixels (same as a 12x12 tileset) and is basically the Grim Fortress character set without the characters, leaving only the tile backgrounds of various transparencies. If you open some tileset -- provided that it's a transparent png -- and put this file below it (in e.g. aseprite, photoshop) then stretch it to the same size as the set, you should have the same tileset but with pretty background colours!
NEXT RELEASE should include:

• the file in different aspect ratios, to require less work (including 1 pixel tiles for simplicity).
• different background shade schemes? Probably limited range for change there.
• an sh/bat script or program for automatic application and/or customisation.