|I don't have the path/file you have.|
[M$ is always moving stuff to create the impression that each version is an 'improvement'. LOL]
Moving right along...
I don't see where you're headed with the findstr and other gyrations, nor why you're using xcopy.
It may help to post the contents of the Custom.theme file.
@echo off & setLocal EnableDELAYedeXpansion
for /f "tokens=* delims= " %%a in ('dir/b/s c:\my.bmp') do (
copy "!src!" d:\dest
Helping others achieve escape felicity