First of all, I want to stress that there is a whole forum dedicated to decals, and this problem has been discussed at length there. Much of these threads should still be available by scrolling down the list of messages.
Colors applied over a dark background will not look very opaque. Clear decal paper is best used for decals used over white and light colors. To create colors to go over dark background, you need to use the white paper. Problem here is that the decal must ordinarily then be cut from the backing with absolute precision.
The way to avoid this is to set the background color for your graphics to the background color you will be applying the decal to. Now, for most background colors this is difficult unless your monitor and printer have been well color-calibrated (not an easy task).
However, black is something else. Setting your graphics background to black works pretty well on most printers. Then, cut as close as possible to the stripes but you do not need to be perfect. The slight black lines you will get around your stripes should blend in reasonably well.