You can enable dark features using either of these methods.
Through system properties
This option is more suitable for someone that manages a large number of instances, as it is a method that can be automated to set these features on startup. First, read the Setting properties and options on startup tutorial for your operative system in the Atlassian documentation site.
The property key that you are enabling must be preceded by "Datlassian.darkfeature".
So, if you want to disable the onboarding experience, which is an Atlassian dark feature, you should locate the property key which in this example would be: "jira.onboarding.feature.disabled". When setting this property through system properties, your key should look like this:
Through the browser
This is an easier way to turn a dark feature on, suitable for admins that manage a single instance. Log in as a user with administrative privileges and paste the following at the end of your site url:
Your URL might look something like https://my-jira-test-site.com/secure/admin/SiteDarkFeatures!default.jspa
From here, simply paste the property key into the text box labelled "Enable dark feature" and click Add.