This has happened in most apps on smartphones, while Apple's MacOS was given the Dark Mode option a few years ago.
But now, Microsoft has also introduced a Dark Mode feature for the Windows 10 operating system to improve the battery life of laptops while keeping the eyes from stressing over longer hours.
You can use Dark Mode as the default look in many Windows apps, as well as many Windows apps, but it is not possible to change some older Windows apps like Task Manager to black.
If you want to use Dark Mode in Windows 10 and its apps, here's the procedure.
To do this, go to the Start menu and open Settings.
Go to the Personalization option there and click on the left side option.
Eliminate the default Windows mode and default App mode from Lite there.
If this is not applicable to an app adapted to the Dark Theme in Windows 10 Settings, you probably need to do so in the app's preferences.
For example, in the Mail app, click Settings at the bottom left and then Personalization, or in the new Edge browser, click Settings and then Appearance in the third dot menu.
There you can adjust the Windows default theme from the app preferences such as standby My Windows mode in the Mail app while the system default option in the Edge browser.