I like it too. Enough so to have documented it in an in-house Jira user guide before realizing that it was not visible to my target users... Argh.
To enable this feature for all users/projects on your instance:
<INSTANCE_URL>/secure/admin/SiteDarkFeatures!default.jspa
where is the first part of the URL that you use to access your JIRA instance.
Then, in the Enable dark feature field, enter
com.atlassian.jira.projects.ProjectCentricNavigation
and finally, click Add
Not sure what the equivalent is for activating just for oneself, unfortunately. Hope this helps.