There are many ways you can theme your android phone. In fact, opportunities are endless if you know how to edit xml files but many of us dont, so here is a guide to theme your android phone.
first of all, you need to know that all system images, Icons etc are stored in apk named Framework-res.apk which is in /system/framework/ folder of your phone, battery icon and status bar elements are stored in SystemUI.apk which is stored in /system/app of your phone.
This guide is a collaboration of very usefull information from many themers/users.
https://docs.google.com/document/edit?id=1-YFrDqBjk7U1w0RIJgvmwH7_famvjxUO9grFR9C15rA&pli=1
Also UOT (ultimate online theming) kitchen is an online service which edits your framework-res.apk and systemUI.apk
This is the best tool to start theming your phone.
Look in Youtube for more info.
UOT Kitchen
I hope you follow all steps properly and dont damage your phone, do proper backup before doing any step, I wont be responsible for any damage