Sans-serif, plenty of whitespace and a single blue accent. This is the theme that shipped compiled into goblog as minimal up to 0.6.x, published as a directory theme so it can be installed and updated on its own.
From your goblog: Admin → Themes → Browse → Minimal → Install, then Activate. Requires goblog 0.7.0 or newer (earlier releases have a built-in minimal that shadows an installed copy).
Upgrading a site that already used the built-in minimal: after the goblog upgrade the site renders the default theme until you install Minimal from the directory; the theme setting is left alone, so the site switches back as soon as the theme is installed.
Only the public-facing templates that differ from default (header, footer, home, post, posts, login and the page_* types) and static/css/goblog.css. Everything else — admin and wizard pages, tags, archives, search, error — renders from goblog's default theme, so it keeps up with new goblog releases automatically.
Edit the files under templates/ and static/, drop the folder into a goblog checkout as themes/installed/minimal/ (or point THEMES_INSTALLED_DIR at its parent) and set the theme setting to minimal. Templates are Go html/template; see goblog.live/docs/writing-a-theme for the contract.
Tag vX.Y.Z and publish a GitHub release; the directory picks up the tag's archive. screenshot.png at the root is shown in the listing.