Everything you write in FolioPress — pages, posts, tags — lives apart from how it is shown. That display layer is a theme, and this guide takes one apart: the folder, the templates, the settings file, and exactly what data each template is handed. If you are comfortable with HTML and CSS you have everything you need.
Swap the theme and the same pages, posts and menu are redrawn — nothing about the content changes, only its outfit. A theme is a folder with three things in it: a settings file, templates, and static assets.
Four templates are required — base.html, home.html, page.html and post.html — and a theme without them will not load. Everything else is optional: a tag archive, a search page, custom error pages, a bespoke layout for one specific page, and a locales/ folder for the theme’s own words. Themes can be edited right in the admin under Themes, exported as a zip, and imported into another site.