You can set darker colors of the Notion backgrounds using CSS variables like so:
:root {
--color-bg-blue: #364954 !important;
--color-bg-red: #594141 !important;
--color-bg-pink: #533B4C !important;
--color-bg-purple: #443F57 !important;
--color-bg-green: #354C4B !important;
--color-bg-yellow: #59563B !important;
--color-bg-orange: #594A3A !important;
--color-bg-brown: #434040 !important;
--color-bg-gray: #454B4E !important;
--color-bg-default: #2F3437 !important;
}