You can't change the background in a KPI object in Sense nor in a text Box
Netherthe less have a look in qlik branch site you will find THEMABLE KPI that allow background color change.
Hope it helps
There is no such formatting options available with the KPI object, but you can always achieve this using Extensions.
Qlik Branch would be the right place for you to get one. If there is no extension which can satisfy your requirement, I would suggest you to build one for your requirement.