Well you would need adb so you can push and pull files from your phone. Then any photo editor, I use photoshop and I have also used photo pos pro which is free, there is also gimp I think and others. What you would have to do is hook your phone up to your pc then use adb to pull your framework go into the res/drawable-hdpi folder and look for statusbar9.png if i recall correctly and edit it to your liking. Then boot into recovery and mount your system, I used to not have to but now i do or it dont work, then use adb and push your framework back. If you need the correct commands and stuff just shoot me a pm and i will give the full details just to much to type out and dont want to overrun the thread.
if you edit a .9 image you will lose the patch. so after you edit it run it through the draw 9 patch tool in your sdk
Then you have to run it through Eclipse to embed the 1 pixel border or else you'll end up with an image with a black pixel around the borders.
In fact, Armyof2, if you want I'll make it for you. let me know what exact theme file you downloaded and are currently running. I'll make a new statusbar_background.9.png for you and load it back into the theme zip file. You can just re-flash it and the status bar will be changed.
You want a gray or white background? Like the stock background?