This theme was created by one of the maintainers of pip, which is where I first saw it (https://pip.pypa.io/en/stable/). Here are some of the things I like about it:

- Well-chosen, proportionate font sizes and spacing.

- Table of contents sidebars for both the current page and the whole documentation site.

- Fully responsive: sidebars disappear in narrow windows or small screens, but can still be popped out.

- Clean color scheme with good contrast, and full support for dark mode (see screenshot at https://github.com/pradyunsg/furo/blob/main/README.md).