It seems the recent Windows 10 Feature update 1809 doesn’t work well with the Windows Explorer for some. Many users are complaining that either the text in the explorer is not white while others are reported that Windows Explorer is still in too much white mode. Some of the users have even reported that drop down dialog boxes are almost invisible. While this could be a bug in the OS, you can try these suggestions and see if it fixes this Dark Mode not working in Explorer problem for you.
Explorer Dark Mode not working
This is known bug for which Microsoft is working to roll out a fix. It seems unexpectedly light-themed UI elements (e.g., address bar) in File Explorer and unreadable text in context menus when using dark theme after the Windows 10 October 2018 Update (1809).
You could try the following:
1] Run System File Checker
We recommend you Run System File Checker in Safe Mode or Boot Time to repair potentially corrupt explorer.exe and other system files.
2] Re-apply the Dark Mode
Next, perform Clean Boot and open Settings > Personalization > Themes
Choose the default theme on your PC and Apply to switch back to the default theme.
Now apply Dark Mode in Clean Boot State and see.
This will fix your problem.
The same fix applies to a similar problem where when viewing a file in the Explorer window the icon labels are in black text and are almost impossible to read against the dark background.
In case nothing is working for you, its best to switch back to the white mode, and wait for Microsoft to roll out an update to fix this problem.
Are you facing similar issues? Let us know in the comments?