The most seamless way to get a true 7-Zip black theme is to use a "fork"—a version of the software where community developers have added the code for dark mode.

Beyond simple "cool factor," a black theme serves a functional purpose in ergonomics and accessibility Reduced Photophobia:

The injector hooks into the 7-Zip process and forces the UI to use dark colors via a DLL injection method. It is safe, lightweight, and does not modify system files.