Aliasing two_tone_bg → two_tone_background
Link to alias

Reason:

Consistency with the other 'bg' shorthand aliased.

Also should alias these:
multicolor_bg -> multicolor_background
multicolour_bg -> multicolor_background
multicolour_background -> multicolor_background
multi_tone_bg -> multicolor_background
multi_tone_background -> multicolor_background
screencap_bg -> screencap_background
3d_bg -> 3d_background