If some things are appearing black and some are white, containers need to be set as primary color in the appearance tab. There is also a boolean in the stylesheet where you can set darkmode to true or false.
The boolean can be found in the theme directiory > styles > native > custom variables. Hope this helps
I thought this was based on your device setting and automatically detected from mendix 8.3 onwards: https://www.mendix.com/blog/mendix-8-3-a-true-delight/
Hope this helps!
Maybe you can use Calypso to toggle to dark mode and customize your theme there?