A page under Library/ tagged meta/theme is a theme: its space-style blocks apply only while it is the active theme, and it shows up in Theme: Select and in Configuration → Appearance → Theme. See Theming for picking themes and Theming/Creating a Theme for writing one.
SilverBullet reads themes from the pages themselves when it starts, so they work before the index is built. Because they are ordinary pages, you can also query them with index.pages("meta/theme").
Besides the usual page attributes, a theme page has these in its Frontmatter:
scheme: light or dark. Required. A meta/theme page without a valid scheme isn’t a theme: its space-style blocks are ignored (its space-lua still runs), and the lint error on scheme says why.displayName: the name shown in pickers. Defaults to the last part of the page name.description: shown in pickers and queries.counterpart: a wiki link to the theme Theme: Toggle Light/Dark switches to.fonts: suggested fonts, with ui, editor, code and editorSize. See Notes for theme authors.The tag has a schema for these attributes, defined in ^Library/Std/Infrastructure/Root Tags, so a theme page with a missing or invalid scheme, or a fonts.editorSize outside 10 to 48, shows lint warnings in its frontmatter. Theme pages get the droplet icon.
The themes in this space:
${query[[ from p = index.pages("meta/theme") order by p.name select {name=markdown.link(p.name), scheme=p.scheme, counterpart=p.counterpart} ]]}