After enabling the dark feature, you need to go to Slack Integration administration page in Jira (in the "add-ons" section) and enable the integration:
At the bottom of this page, there are three URLs, which you will need to use in the Configuring Slack App, to set up Slack event reception and slash commands handlers.
After completing set up of the Slack app, you will need to come back to this page to fill in security information.