Pico.css is an open-source CSS framework for quickly building accessible and elegant web applications. It is unique in the way it prioritizes targeting native HTML elements for styling, rather than using classes and ids which is a more common approach used by popular CSS frameworks such as Bootstrap or Bulma. Pico.css's class-less approach makes it necessary for developers to think more about using correct and semantic HTML structures, leading to more accessible web applications.
It is possible customize Pico.css' colorscheme through CSS variables and the framework's minimalistic approach to styling makes it easy to customize using normal CSS. In their GitHub README, the team behind Pico.css explains that the framework should be used as a starting point, and while its default styles may be sufficient for smaller projects, they encourage that developers use it as a foundation to be built on top of.