This extension is the default category tree layout for Shadows. The CSS is part of /themes/%STORE_ID%/THEME_NAME/extensions.css. If you want to make styling changes, it is recommended you add them to your /themes/%STORE_ID%/THEME_NAME/theme.css file. The HTML for this extension can be accessed by navigating to User Interface -> Global Settings -> Settings -> Category Tree Template.
If you are using Shadows DE, you can modify the CSS in build/extensions/category-tree/ and update as part of your build process.