It's not hard. It is best to edit a theme that's already made. Change colors of icons or change the icons. That way you get an idea of how it all works. Once you get inside the apk, you'll see how a theme works.
All you need to edit is 7zip and paint.net.
Extract the theme files from a theme you wanna play with. (framework-res, systemui and res-cappuccino)
Right click on the apk click 7zip and expand. It will open the insides of the apk. You'll only be messing with the drawables hdpi folder so only extract it. Once you extract it you will see all the icons and images. Edit whichever you want. Save. Once your done right click on the apk again click 7zip expand and go inside the res folder and drag the extracted folder you've been editing inside of over to the 7zip window. It will replace the old folder in the apk.
Push it to your phone and your done. Once you do it you will see that playing around with simple stuff like this ain't hard.
If you even care or wanna learn that is