As of Slack v4+, this theme no longer installs correctly, and some alternatives provided by the community are robust and well-maintained.
On my MacBook, the theme I'm using is https://github.com/elv1n/slack-dark-mojave-theme/blob/master/README.md#slack-dark-mojave-theme
There is discussion, including alternatives that may work for other platforms, at https://github.com/widget-/slack-black-theme/issues/98