Custom CSS is a Firefox plugin designed for web developers and users familiar with CSS, allowing them to inject their own styles into any website. This tool enables users to override default styles, such as font size and color, providing a customizable browsing experience. The subscription-based model ensures continuous updates and support for users, making it a reliable choice for those looking to enhance their web design capabilities.

The plugin is straightforward to use, requiring minimal setup to begin modifying website appearances. With its focus on development and IT, Custom CSS is particularly useful for testing design changes in real-time. By allowing users to tailor their web experience, it serves as an essential tool for web developers aiming to create unique and personalized styles across various websites.