This question was closed as off topic. LightDM login screen configuration
Now LightDM is used with ubuntu and Ubuntu Mate. The DM, greeter, and setting package are available in ubuntu.
The OP had said Debian Stretch, so as it states at the bottom of the close message, I edited the post to fit ubuntu rules: replacing "Debian Stretch" with "LightDM Greeter" since the question is about the greeter. Thus, making it pertain to ubuntu as well.
However, it was closed anyway. Although the OP wasn't using ubuntu, why deprive ubuntu users of the information they may want to customize their login screen?
Edit: A lot of responses seem to be hung on two words that are irrelevant to the question. the OP could have just as easily said "I'm using 16.04 and want to make the Lighdm greeter look like the unity greeter" and his pictures would have been the same. Zana gives the best reason to do anything to the post "there are a lot of answers here about configuring the greeter".
@Zana -- LightDM greeter and Unity greeter are not equivalent. if they were, there wouldn't be two seperte packages in the repo.
For the rest, LightDM and lightDM greeter are available in most linux distros. so how can asking how to customize it be a Debian question??