This has now been merged into Gitea, please see the documentation for instructions on how to enable
A Dark theme for Gitea for Night owls.
Put public/styles.css into the public/ directory of your gitea installation (you may need to create it), and place templates/custom/header.tmpl into the templates/custom/ directory of your gitea installation.
The majority of the CSS from this theme comes from @qdbdbp's User Style Theme.