@piffy Thank you very much. This is a decent workaround indeed.
I got lost in the math of calculating a SVG number that with a 20% opacity will, result in #FAF0FC but this gets close enough to my eyes, and dark mode is dark. Phew!
// Background color instead of Graceful background image
.background-container {
background: rgb(200 190 192 / 20%);
}