Is it possible for black background?

I’m using MKVToolNix to combine SRT files with MP4 or AVI movie files and it all works beautifully!!!
My question…
Is it possible (with either this software or another) to create or make a black background behind the subtitles so they can be seen more easily?
TIA Steve

Figured it out - I’m using Subtitle Edit to do it.
I firstly import a film with subtitles inside an mkv file and then export as a VobSub or BluRay sup with a semi transparent black background.
Then I use MKVToolNix and add that file as a subtitle file and start multiplexing and voila!