By default this theme is not available in the settings, but you can enable it using a registry hack. To do this, open the Registry editor (Win+ R command for run and then type regedit and press enter).
In the registry editor, navigate to the following key.
On the right panel, right click and select New-> DWORD.
In the new DWORD, enter the value name as System Uses Light Theme and enter the value as 0.
Now reboot your computer and it will have the dark theme available. If you want to get back to the original theme, just change the value to 1. We are expecting this feature to come to the final build of Windows 10 when it will be released later this year.