New backgrounds must be defined as BACK01.SCR-BACK17.SCR in your etrasprites. You can look at each of these "back" in ufopaedia site.
Thanks, but since my knowledge about X-com modding is quite limited, I will need help.
In original X-com terror from the Deep and Ufo Enemy Unknown SCR files named BACK01.SCR to BACK17.SCR are located in Geograph file. Not sure how to open them or do with them anything.
In your Mod XCHR I found BACK01, BACK_undead etc.. as png files in Resources/UI folder - these files are I guess backgrounds but how do you force game to use them ? As checking vars.rul I found only
hiddenMovementBackgrounds:
- BACK_INTRUDER
- TAC00.SCR