for simpler steps (no need for decompiling/recompiling)
use Ninjamorph (free on play store)
it allows you to change the PNG files in your apks.
if you do not have 15 toggle mod:
Open Ninjamorph, go to system/app/ then choose SystemUI.apk. Then go to res/drawable-mdpi and then modify the pngs you want to modify (quickpanel_icons)
then after you're done, just go back to main menu of ninjamorph and choose finish then choose systemui.
but if you have 15 toggle mod:
Open ninjamorph, go to system/framework then choose lidroid-res.apk. Then do the same steps.
Ninjamorph is useful for beginners, no need to compile.
EDIT: after you finish you need to reboot (because you edited something in the statusbar, and it also applies if you edited a framework apk.)