Pawel, I am not a "trimmer" either,,,,,,,,,I use the system as it was designed by "someone" back in the sixties. (if I say ModelDecal, it will be Microscale, or MicroMark, or someone)
the film was left around the decal to help with the "layers of ink blending in" when MicroSet and Sol first came out.
one way to work around white numbers and letters is to write them in black, print the background color all around them (either squared off or a random blotch) then change the black areas to clear. This only works if you can get your decal color and paint to match really close.
One way to get them to match is to print out your color on some white film, and match the paint to that,,,,,it is the same as matching a professionally purchased decal to paint colors when you have to (which is fairly often). Print out your color samples and pick you color, then match your paint, then do the drawing with that color added into your program's pallette.
granted, this would be a PITA to do for making white numbers for a Jeep kit, and having to make all of your Olive Drab for that model have to match the decal film before you could paint the model
But, it would work for smaller things,,,,,,,,,like white Happy Hooligans script on the wingtip tanks of your F-89 Scopion.
Rex