Ulbora CMS is a self-contained open-source CMS (no database needed) written in Golang. It uses a JSON datastore with content saved in both json files and in memory. You can download and upload a single binary backup file containing content, images, and templates as needed. There is a built-in mail sender to make it easy to send contact information.
There is a template designer available to make designing templates easy. Individual templates are easy to upload and switch between.