Bulma is a free, open source CSS framework based on Flexbox and built with Sass. It's 100% responsive, fully modular, and available for free.

A UI kit provides developers with a set of reusable components that can be easily integrated into a website or application. These components are pre-designed with consistent styling and functionality, allowing developers to save time and effort in the design and development process. UI kits can be either custom-built or third-party, and often include components for buttons, forms, typography, icons, and more.