If you plan to customise or build upon cu.css, then you will find it much easier to work with the unminified css found in public/css/main.css.
You will find configurations at the top of the file, which can be updated to customise the theme of cu.css. This includes custom webfonts, Utopia fluid type and space scales, colors and other theming related settings. Most of these configurations consist of CSS custom properties which can be updated according to your design preferences.
If you wish to use cu.css without modification, then working with the minified CSS found in public/css/main.min.css will ensure the smallest file size.