You know, putting up a black theme on your desktop environment is not difficult, you can probably find some online for your DE. The problem is the screen of your laptop/PC. Unless your laptop/monitor is a very expensive one, or a Mac, chances are you’re using a cheaper panel. And slapping a 100% black theme there won’t make it as black as you imagine it to be (as in your phone, for example, which usually use good quality screens).