I don't think its possible in 6th theme in non hacky way anyway.
Reaper has folder indentation method tcp_folderindent that puts overlapping images that give this 5th theme folder transition look. As 6th theme has variable indentation amount it doesn't use it, I guess for image scaling issues. Instead it uses hacky code way that cannot use additional images where it wants.
The way I did it is by using Auto Color/Layout script that changes layout for folder tracks to ones that have cutouts in bg images.