What can developers customize on a seller site?

Custom CSS (Branding step, 10000-character limit) can use the variables --ss-accent, --ss-bg, --ss-surface and --ss-text, and each design scopes its styles under the .ss-layout- root class (e.g. .ss-layout-modern). Use Export/Import JSON (Publish step) to back up or migrate the theme and sections between sites, and the Embed section for raw HTML widgets (up to 20000 characters, height 100–1200 px).