Interestingly the calendar app seems to stay in dark mode after a reboot however the email app needs to be re-enabled through developer options if you want it to be dark. I think it's getting more refined folks
Colour defaults back to default theme but if you go into developer options there are now a whole heap extra colours. I changed mine to cinnamon but there is ocean and space and a pink colour too. Cinnamon is a lightish brown colour. But I can't take screenshots.