How to Manual Replace Softkeys?
Ingredients:
-Root Explorer
-Png files for icons.

First, make a folder called softkey and place your favourite soft key inside the folder which is the png. Then copy to your nexus sdcard.
[Note: you need to have 2 types which is example ic_sysbar_recents and ic_sysbar recents_land.]

Open your Root Explorer and go to /system/apk look for systemUI.apk then long press select extract. You will see a popup ask you to go extracted.

Then Back key try go to /sdcard/softkey(or what folder you name...)
Select all the png files.
ic_sysbar_recents and ic_sysbar_recents_land.
ic_sysbar_search and ic_sysbar_search_land.
ic_sysbar_menu_big and ic_sysbar_menu_land_big.
ic_sysbar_back and ic_sysbar_back_land.
Then Move to the folder which you have extracted earlier (the extracted systemui.apk)
/speedsoftware/extracted/systemui

Inside there is lots of files select res then drawable_xdhpi
Paste there, all 8 pngs~

Then only you back till systemui file, long press it select zip it.

It will popup ask you go to zip...

Then long press the zip file, select rename then rename SystemUI.apk

Now you long press select move, move it to /system first

Long press SystemUI.apk, select Change permissions. Change permission into rw-r--r--

Now only you move it into /system/app , paste and it will auto reboot or you do manual reboot

Once you have reboot, you should see you brand new modded soft-keys.