Most of the popular applications have built their UI kits to keep things balanced and uniform. UI design kit is the random set of possible basic UI elements in a design. They also contain experimental UI patterns and many color variations.User interface kits (UI Kits) are the visual language for user interfaces.
Here is a free UI kit. 55 Elements FREE UI KIT is free for download and personal or commercial use..It’s created by Tomasz Mazurczak.