Why does this issue happen?

Here are some causes accounting for it:

1.

Outdated Windows version

2.

How To Enable Windows 10 Dark Mode – Here’s A Detailed Tutorial

Mach2 or other similar tool breaks the legitimate dark mode

3.

Dark theme glitch

Then, lets move to fix the Windows 10 File Explorer dark theme not working issue.

To do that, like refer to the following tutorial.

select Windows from the Change theme part

Step 1:OpenSettingsand then clickUpdate & Security.

Then your Windows will start checking for the available Windows 10 updates and then download them automatically.

Thus, you should probably switch to the default theme.

click Add someone else to this PC

Here is how to do that:

Step 1:Open theSettingsapplication and navigate to thePersonalizationsection.

Step 2:ClickThemesfrom the menu on the left.

In the right pane, selectWindowsfrom theChange themepart.

Step 1:OpenTask Manager.

Step 2:In the pop-up window, find theWindows Explorerprocess.

Right-click it to chooseRestart.

Fix 4: Create a New User Account

Step 1:ClickStartand then clickSettings.

Step 2:ChooseFamily & other usersand then clickAdd someone else to this PCunder theOther userssection.

Step 4:Enter a newUser nameandpassword(optionally), then clickNextto finish creating a new user account.

Step 6:Change theAccount typefromStandard UsertoAdministratorand then clickOKto save changes.

Step 7:Log off from the current user account and log on with the new account.