Hi fujiseed,
Welcome to the Apple Support Communities!
We understand that you're unable to enable Dark Mode on your Mac.
If you've already restarted your Mac, we'd suggest that you create a new user account to test Dark Mode. To create a new user account, you can go to the Apple menu > System Preferences, then click Users & Groups.
Click the lock icon, then enter an administrator name and password.
Click the add button (+) below the list of users.
Complete the fields shown for an Administrator account, then click Create Account (or OK).
Once you’ve created the new account, go to the Apple menu > Log Out, then log in with the new account and attempt to activate Dark Mode.
If you're unable to recreate the same behavior in a new user account, you may consider removing login items from your main account: Remove login items to resolve startup problems on your Mac
We hope this helps.
Cheers!