Shadows, and Shadows-based ReadyThemes, ship with the Search Preview extension included. 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 and JavaScript for this extension can be accessed by navigating to User Interface -> Global Settings -> Search Settings -> Search Preview Settings.
If you are using Shadows DE, you can modify the CSS in build/extensions/search-preview/ and update as part of your build process.