i3wm default setup uses LightDM as a display manager with slick-greeter as a graphical “front-end”.
If you haven’t changed anything during install process, you should have it on board as well.
I don’t remember if slick-greeter uses a GUI settings app, but according to documentation, you should be able to configure it with a config file:
Be aware that the Mint GUI edit front-end for slick-greeter is only available via AUR. You may not want to use it, even though it is a solid piece of software.