You'd have to play around with the theme files. A while ago, I merged two themes to combine the best of both themes. It wasn't a heavy lift, mostly trial and error. I put my recalbox out of service long term until there's support for Pi 4 to install my Pi 3 in my MAME cabinet, otherwise I'd get you the exact code to get rid of the line. I believe you need to set opacity to 0. Despite being a separate system, the retropie wiki has excellent instructions on how to modify the theme files.