Hi
There are many reasons why the dark theme may stop working, but bugs or glitches caused by an older Windows 10 version or outdated apps are the primary causes.
If you don't have time to update your version of Windows 10, you can try one of the solutions below.
- Turn It Off and On Again
- Restart Windows Explorer
- Log Out and Log Back Into Your Account
- Switch Back to Windows 10 Default Theme
- Remove Customization Apps
- Search for Corrupt Files
You can also refer to this document for related steps:
https://www.makeuseof.com/quick-ways-to-fix-the-windows-10-file-explorer-dark-theme
Best Regards,
Wesley Li