What is a Style Kit
A style Kit is a collection of Style Settings, as you control them under the Page Settings > Style tab on the Elementor Editor.
Currently, a Style Kit stores the values of the controls in the Style Panel as shown in the screenshot below. You can Save those Styles as a Style Kit, which is an Entity that you can re-use, maintain and apply on any other Elementor page, or site-wide.
Style Kits as Custom Post Types
Style kits are stored as Custom Post Types, and live under AnalogWP > Style Kits.