CSS is an acronym for Cascading Style Sheets. CSS is used to control the appearance and layout of web pages. For example, you can use CSS to set fonts, colours, margins, line height, width, background images, positioning and many other things.
JRNI allows you to customise the CSS of the booking widget and booking form as an extra feature, allowing you to take complete control of its appearance. To enable custom CSS, follow the instructions here.