SEO, sharing and metadata
Almost everything a search engine or a social platform reads from your site is generated by Ghost, not by the theme. Page titles, meta descriptions, Open Graph and Twitter card tags, canonical URLs, structured data, your sitemap and your RSS feed all come out of Ghost, on any theme, in the same shape.
What the theme contributes is the part that is harder to see and matters as much: clean, semantic markup with one proper heading structure per page, responsive images that keep pages fast on mobile, real paginated archives rather than infinite scroll, and no third-party scripts loading on page one. Page speed and structure are ranking inputs, and they are where a theme can genuinely help or hurt.
The two things fully in your hands, on every post, are the excerpt and the feature image. Between them they decide what appears under your result in search, what appears when somebody pastes your link into a chat, and — on a Ghost site — what your own site search can match against. They take a minute each and most people skip them.