Color screen according to effort

[holocronweaver]

Coming from Bats! HIIT app to HIITMe, I find myself missing how Bats colors the entire screen according to the current stage (red for long rests, yellow for breaks, green for go-time). Coloring the screen is great for someone like me with poor eyesight, this way I can tell from a distance which stage I'm in.

Would you consider allowing a full-screen background color? For now the color could come from the effort level, maybe in the future allow custom event coloring. I could implement it.