You can only change the footer background color and the bottom footer background color, for this you don't need the css but you can adjust it from appearance->customize->footer->footer background color and the bottom footer background color.
As you are suggesting to change the footer background color into two halfs, but its not possible to do so because the structure is not designed accordingly.
If you even try to change the color it will look like the screenshot:
http://prnt.sc/b84njn
So I am sorry but its not possible to change the background color as per your suggestion, as our design structure does not support this.