It's easy to create a theme, you just have to create a new folder. To start using your theme,modify the theme setting in your site's saber-config.yml:

theme: ./src
# or from a npm package
# call `saber-theme-simple`:
theme: simple

A theme should have following folder structure:

├── saber-browser.js # optional
├── saber-node.js    # optional
└── layouts          # required

Theme layouts are populated under $theme/layouts directory and this directory is required, layouts in $projectRoot/layouts will still work and take higher priority.