Invitations, Collages, Slideshows and Scrapbooks – Smilebox Create Custom Google Chrome Theme Online A theme is a special kind of extension that changes the way the google chrome browser looks. It is a zipped bundle of fles: one manifest.json file and several png images Here is an example manifest.json file for a theme: version: One to four dot-separated integers identifying the version of this theme. name: A short, plain text string (no more than 45 characters) that identifies the theme. description: A description text for your theme.