Sierra comes in two different ‘skins’, or colour schemes.
Half Dome is a rather nice green and brown colour scheme (though the function buttons are a little hard to read)
while Glacier Point is mostly grey.
If you want to change the default colour scheme for your login you can do this in
Admin> Settings> Display, choose the skin you want and Save Settings.
You’ll need to log out and back in to see the change.
Which one do you prefer?