Is your feature request related to a problem? Please describe.
Currently when a flash-based ROM is loaded into the RuffleRS Player, it is loaded with a hardcoded backgroundColor value:

https://github.com/rommapp/romm/blob/master/frontend/src/views/RuffleRS/Base.vue#L39

player.load({
  allowFullScreen: true,
  autoplay: "on",
  backgroundColor: "#0D1117", <--------
  openUrlMode: "confirm",
  publicPath: "/assets/ruffle/",
  url: `/api/roms/${rom.value?.id}/content/${rom.value?.file_name}`,
});

While this is fine for most flash ROMs, some have a transparent background that makes it hard to see the game. For example, Bowman 2, as discussed in #1216.

Describe the solution you'd like
The ability to set the backgroundColor attribute. I'd like it to be saved as part of the ROM details itself, but that's probably overkill for such a small edge case.
Instead, perhaps a colour picker in the launch screen for flash ROMs, to allow the value to be overridden for specific ROMs?

Describe alternatives you've considered
I've tried changing the UI modes within both the browser and the ROM, but they have no impact.