I got my Samsung Galaxy s10 today and I'm not impressed because lots of little things seem to be missing. How do I change off of the plain white background when viewing my contacts and messages? I already have a theme installed but I don't see any way to change it for these things.
Not all themes in the Theme Store support dark mode in all the Samsung core apps. If you go back to the default theme you'll be able to turn on dark mode. Or find a different theme in the theme store that supports it. There is also the option of creating your own theme that supports dark mode using the Samsung official app called "Theme Park". You can search for and download it from the Galaxy Store.