Dark Mode

If you mean 12.4.1, then that's because Dark mode is not part of 12.4.1. Its part of iOS 13.

Because there is no iOS 14.1.

Dark mode will be part of iOS 13, which will be released next Thursday.

If you've installed a beta of iOS 13, you should have read the NDA. it can't be discussed here.

Any thing you can add to Control Center can be found in Settings->Control Center->Customize Controls.

If its not there, it cannot be added to Control Center.