Custom CSS is a tool of the Theme Tweaker add-on that aims to change the skin's design elements and add a custom CSS and JS code to the existing X-Cart pages so that the modified code is aggregated with the other software files automatically. Using Custom CSS, you can change the skin colors, fonts, background, etc.
All changes that apply with the Custom CSS tool facilities have a storewide effect (i.e., they affect a store as a whole but not a single page) and record in the Look and Feel > Custom CSS & JS section of your store Admin area.
The tool allows adding custom CSS and JS code in the Admin area. It is also possible to add custom CSS directly from the storefront to see the changes on the fly.