You mean, you change the default theme in Emacs? YES. So if you want a darker theme, you can do that. If you want a light theme, you can do that too. I demonstrate how you can switch to a pre-installed theme, and how you can install a totally new theme into your system.
Once you have installed a theme, it will saved as "safe theme" in your custom file. Where is your custom file? I'm sure we will talk about that in another video.