In the latest v5 theme preview thread, the video talks about how you won’t need two separate themes in order to support dark mode. It shows the if you change a button from blue to say yellow, it automatically will change the text in the button to something with more contrast if needed. (Instead of a yellow button with white text, it would automatically change it to black for example.)
We don’t have full details yet, but it leaves hope the move to a single theme with full dark mode support will handle this better especially since it appears “auto” (match OS) is the default setting.
I would keep an eye on the preview blogs as this MAY already be addressed!