SquareSpace
Squarespace is a website builder designed to help creatives build their own websites and showcase their work, no matter their technical ability.
Themeing
As long as you’re on a higher plan, you can customize JavaScript and CSS.
Developing
Squarespace has developer docs and an API here: developers.squarespace.com
You can clone a base template from Github, and install dependencies with npm.
They also have a local development server solution.
Resources