There are two methods:
1.Using a 7zip,winrar,etc
2. Using an apktool.apkmanager bla bla
If you don't know how to decompile or recompile apk. Just follow the first method :P ( it is easier )
1. 7zip,Winrar, (or any other softwares lke these)
2. Gimp, photoshop ( or anything like these) ---> if you want to edit it
3. Root browser app like Root explorer, Es File Explorer etc.
Now, Using a root browser, go to system/app and copy the SystemUI.apk to your computer.
Open the apk with the program you have.
Then go to res/drawable-ldpi. You can see many png files there.
Now time for you to edit bla bla bla. Make sure you will keep its original name and if you are finished editing.
After that, Put your SystemUI.apk to system/app. Make sure that the permission is right. It is rw-r-r. If it's not. press and hold systemui.apk for a while then an option will appear. Select permissions . In owner. check read and write. In group check read and same with others
(I dont know for the other root browsing applications beacause I'm using root explorer)
Smile and Be Calm. If Force Closes appear. Just reboot it.
1.Knowledge in compiling and decompiling
2.apktool or apk multi tool. (apk multi tool is user friendly but I'm using apktool XD)
3. A root browsing tool.
Using a root browser. Take the SystemUI.apk from system/app
Go to res/drawable-ldpi.
Now edit or change what you want and keep or use the original name of the file
Recompile. Push it to system/app using a root browser and make sure that permissions are rw-r-r.
reboot and enjoy!
Note: If you are on stock rom. You must first deodex it
Don't forget to press the thanks button Hope it helps