Not sure what to say. I thought there was away to format the background, but I am not seeing it.
Normally, it does not appear black after the build - it should be transparent, with a highlight that is set by highlight group.
My quick test was DVD.
If you build to a folder, and play that on the computer, does it show the problem?
If you create a new subtitle track, add a manual subtitle, does it do the same thing?
