The background is derived of the tertiary colour, #595bca in the below example
--background-color: light-dark(oklch(from #595bca 96% calc(c * 0.125) h), oklch(from #595bca 10% 0.025 h)) !important;
Defined in this file _color_definitions_entrypoint.scss
If you need it to be a different shade, you’d have to tweak the values; changing 96% → 98% for example will produce a lighter pink