SEO

Automatic search-engine plumbing: structured data, Open Graph and metadata - on by default, one switch to turn it off.

What the theme does automatically

When stygian.seo.enabled is true (the default) every page gets:

WebSite JSON-LD in the <head> - site name, optional alternateName from site.tagline, and the site URL
BreadcrumbList JSON-LD on every docs page - matches the visible breadcrumbs (Home, parent page when nested, current page)
Open Graph tags - og:title, og:description, og:url, og:type, og:site_name
Twitter card - summary_large_image when an image is configured, summary otherwise
Meta description - page.description, else page.lede, else site.description, truncated to 160 characters
Canonical URL - site.url + site.baseurl + page.url, always

No plugin is required: it is all Liquid in the layouts.

Adding an Open Graph image

stygian:
  seo:
    enabled: true
    image: /assets/img/og.png

Relative paths are resolved against site.url and site.baseurl; an absolute URL is used as-is. GitHub Pages commonly hosts the image in the site itself, e.g. https://user.github.io/project/assets/img/og.png.

Turning it off

stygian:
  seo:
    enabled: false

This removes JSON-LD, Open Graph and the Twitter card. The title tag, meta description and canonical link stay - they are core meta.

Good hygiene around SEO

Set url and baseurl in _config.yml. JSON-LD and canonical URLs are absolute only when site.url is set.
Keep one H1 per page. Stygian renders the page title as the H1 and strips a redundant leading # Heading from the markdown content, so write content headings from ## upwards.
Use lede or description front matter on pages whose first paragraph is not a good meta description.
Add jekyll-sitemap to your plugins: list for a sitemap.xml; add jekyll-seo-tag only if you want its extra niceties - the theme does not need it.
Check the result: page source shows the JSON-LD blocks, and Google’s Rich Results test understands BreadcrumbList.