if you want your background (main) to be translucent while keeping everything else at 100 opacity you can just use a rgba color picker and set the level of opacity you want. for example
main {
background: rgba(97, 16, 58, 0.5);
}
if you want anything more complex (frosted glass look, blurryness, etc than you can spesify)
rgba colorpicker if you can't click inserted links: https://rgbacolorpicker.com/