Ultrachromic is a CSS theme for the Jellyfin media server that provides a modular system of CSS overrides to customize the appearance of the Jellyfin web interface. It consists of multiple CSS files that can be imported in a specific order to build a theme, including base styling, accent colors, rounding, progress bars, and various visual effects. The theme uses CSS variables for easy customization and includes presets that replicate the appearance of older chromic themes. It demonstrates good use of modern CSS features such as CSS variables, animations, backdrop-filter, and media queries.