If there are custom ROMs available for your phone some of those may have different system icons, or give you the option to "theme" them. But again, we're talking about rooting the phone.
I've rooted and modified devices, though not for a few years, and I have done all of the above (including manually changing icons in framework-res and systemUI). It's doable, but I would strongly recommend that you learn what rooting means, not just "where's there a recipe I can follow", before deciding whether it's something you want to do. It gives you more control, but it requires you take more responsibility, it may break apps that you regard as essential, it will probably void your warranty, and it may not be possible at all (it depends on what device you have). Do it only if you understand what it involves. And it's very easy to mess up when you alter system apps like this: the first "bootloop" I ever suffered (i.e. damage to system software that prevented the phone from starting up) came from doing just this, theming the system icons! (And that was using a tool that was supposed to make it easy, by the way).