Create a transparent image in Photoshop or GIMP the size of your header, save it as a .png, and upload it via the media gallery.
Then, use some custom css to define that image as the background for the header, making sure to add !important
I haven’t tested it, but this is the only way I can think of making this transparent. (Header images are .jpg by default, and .jpg doesn’t support transparent images.) YMMV