CSS stands for Cascading Style Sheets, and it is a programming language used to describe the presentation and layout of web pages written in HTML or XHTML.
In WordPress, CSS is used to customize the appearance of the theme, such as changing the font, color, size, and layout of various elements on the website.
CSS code can be added to WordPress in several ways, including using the built-in theme customizer, adding custom CSS to the theme’s style.css file, or by using a plugin that allows for custom CSS.