Adds a GitHub-style theme (light, dark and auto) built from Primer Primitives and Octicons, with a few template overrides. One package works on both Gitea 1.27.x and 28.x. MIT licensed.
Named `mechiland/GitHub` to tell it apart from the other GitHub-style entries, following the `pat-s/GitHub` pattern.
Reviewed-on: https://gitea.com/gitea/awesome-gitea/pulls/201
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: mechiland <246922+mechiland@noreply.gitea.com>
